Mgf2 optical thin film including amorphous silicon oxide binder, optical element provided with the same, and method for producing mgf2 optical thin film

ABSTRACT

An MgF 2  optical thin film is formed on an optical surface of a base material. The MgF 2  optical thin film includes MgF 2  particles and an amorphous silicon oxide-based binder which exists on the surfaces of the MgF 2  particles and between the MgF 2  particles. Owing to this amorphous silicon oxide-based binder, the optical thin film can have high mechanical strength and high adhesion to the base material, while having excellent environment resistance and a lower refractive index.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/968,268 filed Dec. 14, 2010, which is a division of U.S. patentapplication Ser. No. 11/662,892 filed Mar. 15, 2007, which is a 371 ofInternational Patent Application No. PCT/JP2005/017015 filed Sep. 15,2005.

TECHNICAL FIELD

The present invention relates to a singlelayered or multilayered opticalthin film formed of MgF₂, a method for producing the same, an opticalelement having the MgF₂ optical thin film, and an optical system foroptical instruments such as cameras, microscopes, binoculars, andexposure apparatuses which is provided with the optical element.

BACKGROUND ART

Surfaces of individual lenses, which construct an optical system such asa camera lens and an objective lens of a microscope, are coated with anantireflection film in order to reduce the reflection. In general, theoptical thin film such as the antireflection film is produced by the drymethod (dry process), in which the vacuum vapor deposition method, thesputtering method, the CVD method (Chemical Vapor Deposition) or thelike is used.

In order to obtain a high performance optical thin film having a lowreflectance in a wide wavelength band or a wide angle band, it is knownthat a multilayered film is appropriately formed by combining aplurality of coating materials having different refractive indexes.Usually, when an antireflection film is formed by the dry process, thenTiO₂ (refractive index: 2.4 to 2.7 at 500 nm) is usually utilized as themaximum refractive index material, and MgF₂ (refractive index: 1.38 at500 nm) is utilized as the minimum refractive index material.

The following fact is known for the multilayered film. That is, when thedifference in the refractive index between the coating materials to beused is larger, or when a low refractive index film is used at theuppermost layer, then the optical performance is improved, and/or thenumber of coating layers can be decreased even when the opticalperformance is same. In particular, it has been clarified by thesimulation that the optical performance can be extremely enhanced, whenonly the uppermost layer is composed of a low refractive index filmhaving a refractive index of not more than 1.30. That is, the lowrefractive index film, in which the uppermost layer has a refractiveindex of not more than 1.30, is effective for the realization of thewide band in which the reflectance can be suppressed to be low over awide wavelength region. Further, the low refractive index film is alsoextremely effective for the realization of the wide incidence in whichthe reflectance can be suppressed to be low not only for the direct ornormal incoming (incident) light but also for the light allowed to comein a wide angle range. Therefore, a technique is required, in which theoptical thin film having a refractive index of not more than 1.30 can beproduced.

In order to lower the refractive index of the film, it is effective thatthe structure of the film is porous rather than dense. In general, it isdefined that the film has a structure of a plurality of minute holes orpores which separate the deposited solid substance. Therefore, therelationship between the packing density and the refractive index of thefilm is as follows.

n _(f) =n _(o) ×P+n _(p)×(1-P)

In this expression, n_(p) represents the refractive index of thesubstance (for example, air or water) with which the minute holes arefilled, n_(f) and n_(o) represent the actual refractive index (dependingon the packing density) and the refractive index of the deposited solidmaterial respectively, and P represents the packing density of the film.Further, the packing density is defined as follows.

P=(volume of solid portion of film)/(total volume of film (solidportion+minute hole portion))

Thus, the high and low the packing densities mean the high and low therefractive indexes respectively.

In general, the dry process such as the vapor deposition and thesputtering is suitable in order to obtain the dense film. However, thewet method (wet process) is suitable in order to obtain the porous film.The wet process is such a method that the film is formed by coating thesubstrate with the liquid by, for example, the spin coat method, the dipmethod, the spray method, and the roll coat method, followed by beingdried and heat-treated. The feature of the wet process is exemplifiedsuch that any large-sized apparatus is not required, unlike the dryprocess, and that the film can be formed in the atmospheric air.Therefore, it is possible to greatly lower the cost. For example, in thecase of the lens having a small radius of curvature, it is difficult touniformly effect the coating of the optical thin film by the dry processsuch as the vacuum vapor deposition method and the sputtering method.However, the uniform coating can be performed relatively easily in thecase of the wet process such as the spin coat method. In this case, thefilm can be formed uniformly on a surface having a large areal size andon a curved surface having a small radius of curvature as well.

International Publication No. 02/18982A1 discloses a method forproducing a porous MgF₂ film by the wet process. In this method, a solsolution of MgF₂ is heat-treated at a high temperature and a highpressure to thereby effect the grain growth and the crystallization ofMgF₂ minute particles, which is thereafter subjected to the coating toform the film. According to this method, even when the film is formed bydepositing the MgF₂ minute particles, the pores, which exist between theminute particles, are not crushed, and the high porosity is secured. Asa result, the film is porous. It is possible to extremely lower therefractive index as compared with any dense film produced by the dryprocess. However, the following problem arises. That is, the mechanicalstrength of the obtained porous film is low, and the adhesive force islow with respect to the substrate, and that when the manual wiping isperformed, the porous film is exfoliated.

A large number of techniques are known in order to improve the filmstrength and the adhesive force of the porous films based on the use ofvarious types of minute particles. For example, Japanese Patent No.3272111 discloses a technique for reinforcing or enhancing an antistaticfilm composed of SnO₂ minute particles with which a surface of a cathoderay tube is coated. In this technique, a sufficient strength is given toan SnO₂ film by forming an SiO₂ film on the SnO₂ film by the wetprocess. However, the refractive index is not lowered sufficiently,because the dense SnO₂ film is formed at the uppermost layer.

A technique is disclosed in Japanese Patent Application Laid-open No.11-6902 as an example of the techniques for reinforcing a porous filmitself, in which the porous film composed of inorganic minute particlesis reinforced with a polymer binder. In this technique, it is possibleto reinforce the film itself. However, the refractive index of the filmcannot be lowered to be not more than 1.30, because the refractive indexof the polymer is relatively high.

Japanese Patent Application Laid-open Nos. 7-48527 and 8-122501 disclosea technique in which a porous film composed of SiO₂ minute particles isreinforced with a binder of alkoxysilane. The film itself can be alsoreinforced in the case of this technique. However, SiO₂ has a propertyto easily adsorb the water content in the air. Further, the film isporous, which has a large surface area. For this reason, the largewavelength shift is caused. Therefore, the film can be used as anantireflection film for display devices. However, it is difficult to usethe film for any precision optical instrument such as cameras,microscopes and the like.

There is such a possibility that the wavelength shift can be suppressedby using a sol described, for example, in Japanese Patent ApplicationLaid-open No. 2000-169133. This document describes, as a coating agent,the sol of composite colloid particles of 5 to 50 nm in which colloidalsilica and MgF₂ hydrate are coagulated. Although there is no descriptionabout the film in Japanese Patent Application Laid-open No. 2000-169133,when any film is formed, the wavelength shift is hardly caused becauseMgF₂ has a property to hardly adsorb the water content. However, in thecase of the sol as described above, it is not necessarily affirmed thatthe sol is excellent in the environment resistance, because the solcontains the unstable MgF₂ hydrate which is not pure MgF₂.

In recent years, the optical system is increasingly complex andversatile, as the required performance is enhanced. For example, thenumber of lenses is increased, for example, in order to maximally chasethe aberration to the limit or in order to increase the zoommagnification. It is also necessary to provide such a design that theangle of incidence of the light beam into the lens surface is increased.Further, as the digital camera comes to the front in recent years, forexample, the element, which has been the film, is progressively replacedwith the image pickup device such as CCD and CMOS.

When the change of the optical system is assessed from a viewpoint ofthe surface reflection of the lens or the like, the increase in thenumber of lenses is directly the increase in the number of reflectingsurfaces. The antireflection film is applied in ordinary cases. However,the possibility is increased that the ghost and flare are caused due tothe residual reflection, and the transmittance is lowered as well. Asfor the increase in the angle of incidence, in principle, there is sucha tendency that the surface reflection is increased as the incidence iseffected more obliquely, irrelevant to the presence or absence of theantireflection film, which makes the cause of the ghost and the flare.The reflection of the image pickup device has not been hithertoconsidered. It is pointed out that the reflected light is returned tothe optical system to cause the flare and the ghost. The ghost and theflare cause the decrease in the contrast and the deterioration of thecolor tone, and they cause the disappearance of the image in the worstcase, which are of course unfavorable.

The antireflection film, which is generally used at present, isinitially a singlelayered antireflection film. However, thesinglelayered antireflection film is changed to the multilayeredantireflection film in order to widen the wavelength band or zone. Asthe production technique is improved, the antireflection film issufficiently investigated and contrived, for example, such that theantireflection characteristic is adjusted. The optical design isprogressively contrived as well, for example, such that the angle ofincidence is restricted or limited so that various problems are notcaused, in consideration of the proper arrangement of the antireflectionfilm. As a result, a lens, which involves less problems to some extent,is completed (see Japanese Patent Application Laid-open No. 62-124503).

However, such a lens is established on the sacrifice of the degree offreedom of the optical design. As the high performance is requiredand/or the new element or device such as CCD is used as described above,it is recognized that the performance of the conventional antireflectionfilm is insufficient.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the case of the film using MgF₂ as described inInternational Publication No. 02/18982A1, the film strength and theadhesive force with respect to the base material are insufficient,although the refractive index of the optical thin film can be lowered tobe not more than 1.30. In the case of the films in which the filmstrength and the adhesive force with respect to the base material can besecured as described in Japanese Patent No. 3272111, Japanese PatentApplication Laid-open No. 11-6902, Japanese Patent Application Laid-openNo. 7-48527, Japanese Patent Application Laid-open No. 8-122501 andJapanese Patent Application Laid-open No. 2000-169133, the problemarises such that the refractive index of the film cannot be loweredsufficiently, and the environment resistance is insufficient. There hasbeen the problem such that it is not possible to obtain any optical thinfilm which is capable of sufficiently decreasing the refractive indexand of securing the mechanical strength of the film, the adhesive forcewith respect to the base material, and the environment resistance.

When the dry process such as the vapor deposition method is used, thecoating can be performed with the multilayer antireflection film inwhich high refractive index films and low refractive index films arealternately stacked. However, in the case of the wet process, it isdifficult to perform the multilayered coating. Therefore, when the filmis formed by the wet process, the antireflection film having the singlelayer is generally formed. In the case of the single layer, thereflectance can be theoretically made 0% when the refractive index ofthe film is the square root of the refractive index of the substrate.

Therefore, if the refractive index of the antireflection film can befinely changed for a variety of optical glasses having differentrefractive indexes, it is possible to produce, for example, a cameralens and an objective lens having the excellent performance. A techniqueis required, in which the refractive index of the film is adjusted inresponse to the materials of the lenses having various refractiveindexes.

In order to adjust the refractive index of the film, the followingprocedure is easily adopted. That is, a film, which is porous and whichhas a low refractive index, is used as a base, and the porous film isdensified so that the refractive index is adjusted to be high. On thecontrary, if it is intended to lower the refractive index by providing aporous property to a film which is dense and which has a high refractiveindex, there is such a high possibility that the film itself may becollapsed (destroyed). Therefore, such a procedure is extremelydifficult to be executed. Therefore, a porous film, in which therefractive index is sufficiently lowered, is especially required inorder to adjust the refractive index as well.

In the case of the film using MgF₂ as the porous film in which therefractive index is sufficiently lowered as described in InternationalPublication No. 02/18982A1, it is possible to improve the bonding forcebetween the minute particles so that the strength of the film itself andthe adhesive force with respect to the substrate can be improved byheat-treating the porous MgF₂ film at a high temperature. However, noremarkable effect can be obtained unless the heat treatment is performedat a temperature of not less than 300° C. Therefore, the followingpossibility arises. That is, the film can have a dense property due tothe heat treatment of the porous film at the high temperature, therefractive index can be increased, and the fluorine of the fluoride maybe liberated and changed into any oxide. Further, when the substrateundergoes the high temperature, then the accuracy of the opticallypolished surface is changed, the refractive index is changed, and thesubstrate is broken in some cases when the substrate has the low heatresistance.

The realization of the wide band can be achieved at most by introducingthe multilayered structure, in the case of the design and the processhaving been hitherto used for the optical thin film. In suchcircumstances, it is almost impossible to absolutely realize the lowreflectance and reduce the reflection at high angles of incidence whilemaintaining the band.

In view of the above, a first object of the present invention is toprovide an MgF₂ optical thin film which has the high mechanicalstrength, which has the high adhesive force with respect to a substrate,which is excellent in the environment resistance, and which provides thelower refractive index with ease.

A second object of the present invention is to provide a productionmethod in which such an MgF₂ optical thin film can be produced with easeand to provide an MgF₂ sol solution which is suitable for the productionmethod.

Additionally, a third object of the present invention is to provide amethod for producing an MgF₂ optical thin film in which the refractiveindex of the MgF₂ optical thin film can be adjusted with ease.

Further, a fourth object of the present invention is to provide amultilayered antireflection film, an optical element, and an opticalsystem in which the absolutely low reflectance can be realized and thereflection can be reduced at higher angles of incidence whilemaintaining the realization of the wide band.

Means for Solving the Problem and Effect of the Invention

According to a first aspect of the present invention, there is providedan MgF₂ optical thin film, comprising MgF₂ minute particles; and anamorphous silicon oxide-based binder which exists between the MgF₂minute particles.

According to the present invention, the MgF₂ minute particles, which areexcellent in the environment resistance (durability), are used as themain constitutive substance of the film, and the MgF₂ minute particlesare bonded or connected to one another by the amorphous siliconoxide-based binder. Therefore, the strong bonding or connection can beprovided between the MgF₂ minute particles and between the MgF₂ minuteparticles and the base material. The mechanical strength of the film andthe adhesive force between the film and the base material are improved.That is, it is considered that the MgF₂ optical thin film has such astructure that the voids (gaps) are present between the MgF₂ minuteparticles, and the voids are filled with the amorphous siliconoxide-based binder. SiO₂, which is somewhat inferior in the environmentresistance, can be used as the amorphous silicon oxide-based binder.However, it is enough to use a small amount of the binder as comparedwith the MgF₂ minute particles. Therefore, it is possible tosufficiently secure the environment resistance as the entire thin film.

In the MgF₂ optical thin film of the present invention, the MgF₂ minuteparticles may be connected by the amorphous silicon oxide-based binder,and the amorphous silicon oxide-based binder, disposed on surfaces ofMgF₂ minute particles, among the MgF₂ minute particles, which exist atan outermost portion of the MgF₂ optical thin film, may have a thicknesswhich is not more than 5% of a wavelength of light to be radiated.Further, in the MgF₂ optical thin film of the present invention, theamorphous silicon oxide-based binder, which exists between adjacent MgF₂minute particles, among the MgF₂ minute particles, may have a thicknesswhich is smaller than a particle diameter of the MgF₂ minute particles.When the thickness of the binder is adjusted as described above, it ispossible to lower the refractive index. In particular, when therefractive index of the outermost layer of the optical thin film issufficiently lowered, the antireflection film is obtained, which has thelow reflectance in a wide wavelength band or zone and in a wide angleband or zone.

In the present invention, it is unnecessary that the amorphous siliconoxide-based binder exists in all of the spaces between the MgF₂ minuteparticles. It is enough that the amorphous silicon oxide-based binderexists in a part of the spaces between the MgF₂ minute particles tomaintain the thin film of the MgF₂ minute particles. It is unnecessarythat the amorphous silicon oxide-based binder completely exists in thespaces between the MgF₂ minute particles and the base material. It isenough that the amorphous silicon oxide-based binder exists in a part ofthe spaces between the MgF₂ minute particles and the base material, andthus the amorphous silicon oxide-based binder reinforces the bondingbetween the MgF₂ minute particles and the base material.

In the MgF₂ optical thin film of the present invention, the MgF₂ minuteparticles may have an average particle diameter of 1 nm to 100 nm (notless than 1 nm and not more than 100 nm). In this case, owing to thehigh crystallization property of the MgF₂ minute particles, the adhesionor agglutination between the MgF₂ minute particles can be suppressed tosecure the voids between the MgF₂ minute particles so that the MgF₂optical thin film having a porous structure may be successfullyobtained. When the porous structure is provided, it is possible toobtain the MgF₂ optical thin film having the lower refractive index. Theporous structure may have a percentage of voids or porosity of not morethan 50%, in view of the retention of the film strength.

In the MgF₂ optical thin film of the present invention, the amorphoussilicon oxide-based binder may be formed of amorphous silica. In thiscase, the refractive index of amorphous silica is low, i.e., 1.42.Therefore, it is possible to secure the low refractive index of theentire thin film even when the composite is formed together with theMgF₂ minute particles.

The MgF₂ optical thin film of the present invention may have a strengthof the thin film of not less than 30 MPa and especially not less than110 MPa, as measured by a microindentation test method. In this case, noscratch is formed even when the film surface is manually wiped, becausethe film strength is not less than 30 MPa. The MgF₂ optical thin film iseasily applied to a variety of ways of use.

The MgF₂ optical thin film of the present invention may have arefractive index of 1.10 to 1.42 at a design center wavelength λ_(C). Inthis case, the refractive index of the MgF₂ optical thin film is withinthe predetermined range. Therefore, it is easy to form theantireflection film by using the MgF₂ optical thin film as a lowrefractive index material.

A multilayered optical thin film of the present invention comprises aplurality of stacked optical thin films, wherein the MgF₂ optical thinfilm of the present invention may be stacked as an outermost layer ofthe stacked optical thin films. The refractive index of the outermostlayer can be suppressed to be sufficiently low, because the MgF₂ opticalthin film of the present invention is stacked at the outermost layer inthe multilayered optical thin film of the present invention. It ispossible to obtain the antireflection film having the low reflectance ina wide wavelength band and a wide angle band.

A multilayered optical thin film of the present invention comprises aplurality of stacked optical thin films, wherein a plurality of MgF₂optical thin films, each of the films being of the present invention,are included in the multilayered optical thin film. In this case, otherlayers can be stacked, because the mechanical strength of the MgF₂optical thin film is high. The range of application of the MgF₂ opticalthin film is wide. In the multilayered optical thin film of the presentinvention, a plurality of MgF₂ optical thin films may be disposedadjacently to each other, and a difference in refractive index betweenthe adjacent MgF₂ optical thin films may be 0.02 to 0.23.

A multilayered optical thin film of the present invention comprises aplurality of stacked optical thin films, wherein the stacked opticalthin film may include the MgF₂ optical thin film of the presentinvention and an optical thin film formed by a dry process.

An optical element of the present invention comprises a base materialwhich has a refractive index of 1.4 to 2.1; and the MgF₂ optical thinfilm of the present invention which is stacked on at least one ofoptical surfaces of the base material; wherein at least one of theoptical surfaces is formed to have one of a flat surface and a curvedsurface.

In the optical element of the present invention, at least one of theoptical surfaces of the base material may be formed to have the curvedsurface form having such a shape that (effective lens diameter D)/(lensradius R) is 0.5 to 2. In this case, according to the optical element ofthe present invention, the MgF₂ optical thin film can be formed by thewet process. Therefore, the MgF₂ optical thin film can be formed to havea uniform thickness entirely on the optical surface even when the thinfilm is formed on the curved surface having D/R within the predeterminedrange. Therefore, it is easy to obtain the excellent opticalcharacteristic.

An optical element of the present invention comprises a substrate, and amultilayered antireflection film which is formed on the substrate andwhich is constructed of a stack of at least three types of layers havingdifferent refractive indexes respectively; wherein an uppermost layer,of the multilayered antireflection film, which makes contact with amedium, may be the MgF₂ optical thin film of the present invention, theMgF₂ optical thin film having a refractive index of not more than 1.30at a design center wavelength λ₀; and remaining layers, of themultilayered antireflection film, other then the uppermost layer may beconstructed by stacking a layer having a refractive index of not lessthan 2 at the design center wavelength λ₀ and a layer having arefractive index of 1.38 to 1.7 at the design center wavelength λ₀. Inthis case, the wavelength band characteristic or the incident anglecharacteristic is remarkably improved. The reflectance can be suppressedto be low with respect to the light beam allowed to come in a wide anglerange, and the reflectance can be suppressed to be low over a widewavelength region.

In the optical element of the present invention, a layer, among thelayers, which makes contact with the substrate, may have a refractiveindex of 1.38 to 1.7 at the design center wavelength λ₀; and a secondlayer counted from the medium may have the refractive index of not lessthan 2 at the design center wavelength λ₀. When the refractive indexesare adjusted as described above, the reflectance can be furthersuppressed to be low over a wide wavelength region.

It is possible to obtain the optical element which makes it possible tosuppress the reflectance to be low.

In the optical system of the present invention, Rn×Rm≦0.002% may besatisfied (in the entire visible region) provided that Rn represents areflectance of normal incidence on an n-th ghost-generating surface inthe optical system, and Rm represents a reflectance of normal incidenceon an m-th ghost-generating surface. When this relationship issatisfied, it is possible to obtain an image in which the ghost and theflare are more suppressed with the optical system.

In the optical system of the present invention, the multilayeredantireflection film of the present invention may be applied to at leastone of the n-th and m-th ghost-generating surfaces. In this case, it ispossible to obtain an image in which the ghost and the flare are furthersuppressed with the optical system.

In the optical system of the present invention, the multilayeredantireflection film may be applied to a surface to which a flat surfaceor a concave surface is opposite as viewed from a diaphragm of theoptical system. In this case, it is possible to more effectively obtainan image in which the ghost and the flare are further suppressed withthe optical system. In other words, if the reflection is caused on thesurface to which the flat surface or the concave surface is opposite asviewed from the diaphragm of the optical system, the influence isgreatly exerted on the image as compared with a case in which thereflection is caused on any other surface. Therefore, when themultilayered antireflection film is provided on the surface as describedabove to suppress the reflection, it is possible to obtain an image inwhich the ghost and the flare are further suppressed more effectively ascompared with a case in which the multilayered antireflection film isprovided on any other surface.

The optical element of the present invention may be used for a lightbeam having a wavelength region of 400 nm to 800 nm. The optical elementof the present invention may be used for an imaging optical system or anobservation optical system.

The optical system of the present invention is constructed of aplurality of optical elements arranged between an object and an imageplane, wherein at least one of the plurality of optical elements is theoptical element of the present invention.

According to a second aspect of the present invention, there is provideda method for producing an MgF₂ optical thin film, comprising a step ofpreparing a sol solution in which MgF₂ minute particles are dispersed; astep of preparing a binder solution which contains a component capableof forming an amorphous silicon oxide-based binder by a reaction; a stepof preparing a coating liquid by mixing the sol solution and the bindersolution; a step of forming a film by coating the coating liquid on abase material and by performing drying; and a step of performing a heattreatment after forming the film.

The optical thin film of the present invention as described above can beproduced by the method for producing the MgF₂ optical thin film of thepresent invention. According to this production method, the sol solutionand the binder solution are mixed with each other to prepare the coatingliquid, and the coating liquid is coated on the base material, then byperforming drying to form the film. Therefore, it is possible to coatthe sol solution and the binder solution together on the base material.The labor, which is required for the coating and the drying to form thefilm, is decreased. It is thus easy to produce the MgF₂ optical thinfilm with which the effect is obtained as described above.

According to a third aspect of the present invention, there is provideda method for producing an MgF₂ optical thin film, comprising a step ofpreparing a sol solution in which MgF₂ minute particles are dispersed; astep of preparing a binder solution which contains a component capableof forming an amorphous silicon oxide-based binder by a reaction; a stepof forming a porous film by coating the sol solution on a base materialand by performing drying; a step of coating the binder solution on theporous film and impregnating the binder solution into the porous film;and a step of performing a heat treatment after the impregnation.

According to the method for producing the MgF₂ optical thin film of thepresent invention, the sol solution is coated on the base material,followed by performing drying to form the porous film. The porous filmis coated and impregnated with the binder solution. Therefore, there isno labor to uniformly or homogeneously mix the sol solution and thebinder solution. Further, the respective solutions are not mixed witheach other. Therefore, the interaction is scarcely caused between thecomponents of the respective solutions. Accordingly, it is easy toselect the respective components and it is easy to produce the MgF₂optical thin film with which the effect is obtained as described above.Therefore, it is appropriate to select the production method accordingto the second or third aspect of the present invention depending on thesolution components.

In the method for producing the MgF₂ optical thin film of the presentinvention, the sol solution may be prepared by synthesizing the MgF₂minute particles by reacting a magnesium compound and a fluorinecompound in a solvent. Accordingly, it is possible to prepare the solsolution in which the MgF₂ minute particles are dispersed uniformly orhomogeneously.

In the method for producing the MgF₂ optical thin film of the presentinvention, the sol solution may be prepared by mixing the magnesiumcompound and the fluorine compound in the solvent and performing atleast one of a pressurizing treatment and a heat treatment. Accordingly,it is easy to prepare the sol solution in which the more crystallineMgF₂ minute particles are dispersed uniformly or homogeneously.

In the method for producing the MgF₂ optical thin film of the presentinvention, the magnesium compound may be magnesium acetate, the fluorinecompound may be hydrofluoric acid, and the solvent may be methanol.

In the method for producing the MgF₂ optical thin film of the presentinvention, a molar ratio of fluorine contained in the fluorine compoundexisting in the solvent to magnesium contained in the magnesium compoundexisting in the solvent may be 1.9 to 2.0.

In the method for producing the MgF₂ optical thin film of the presentinvention, the component, which is capable of forming the amorphoussilicon oxide-based binder, may be an organic silicon compound. When theorganic silicon compound is used, SiO₂ can be formed by the reactionbetween the MgF₂ minute particles. Therefore, the connection can be madebetween the MgF₂ minute particles with a small amount of the binder.

In the method for producing the MgF₂ optical thin film of the presentinvention, the organic silicon compound may be silicon alkoxide, apolymer thereof, or polysilazane. When the compound as described aboveis used, the reaction can be performed at a lower temperature to effectthe connection between the MgF₂ minute particles.

In the method for producing the MgF₂ optical thin film of the presentinvention, an SiO₂-converted concentration of silicon in the coatingliquid or the binder solution to be coated on the porous film may be notmore than 5% by weight. Accordingly, it is possible to make theconnection between the MgF₂ minute particles with a smaller amount ofSiO₂.

In the method for producing the MgF₂ optical thin film of the presentinvention, the coating liquid or the sol solution may be coated on thebase material by a spin coat method or a dip coat method. When themethod as described above is used, it is easy to form a more uniformMgF₂ optical thin film.

In the method for producing the MgF₂ optical thin film of the presentinvention, the coating liquid or the sol solution may be coated on thebase material in an atmosphere of relative humidity of 5% to 40% by aspin coat method. The present inventors have found out the followingfact. That is, when the coating is performed at the specified relativehumidity as described above, then any unevenness such as any radialstripe is hardly caused during the coating, and it is possible to form amore uniform MgF₂ optical thin film.

In the method for producing the MgF₂ optical thin film of the presentinvention, the coating liquid or the sol solution may be coated on thebase material by a spin coat method by rotating the base material at amaximum number of revolutions of not less than 500 rpm and not more than9,000 rpm within 0 second to 3 seconds after supplying the coatingliquid or the sol solution to the base material. When this procedure isadopted, then any unevenness such as any radial stripe is hardly causedduring the coating, and it is easy to form the more uniform MgF₂ opticalthin film.

In the method for producing the MgF₂ optical thin film of the presentinvention, the MgF₂ optical thin film having a desired refractive indexmay be produced by adjusting an SiO₂-converted concentration of siliconin the binder solution or the coating liquid which is to be coated onthe porous film and with which the porous film is to be impregnated. Inthe method for producing the MgF₂ optical thin film of the presentinvention, wherein a plurality of pieces of the MgF₂ optical thin filmhaving desired refractive index may be produced by adjusting a molarratio of fluorine contained in the fluorine compound to magnesiumcontained in the magnesium compound of the sol solution. The refractiveindex of the MgF₂ optical thin film to be obtained can be adjusted byadjusting the concentration of silicon of the binder solution or thecoating liquid and/or adjusting the F/Mg ratio of the sol solution.Therefore, it is easy to produce the MgF₂ optical thin film having thedesired refractive index. The MgF₂ minute particles may have an averageparticle diameter of 1 nm to 100 nm.

A binder-containing MgF₂ sol solution of the present invention is a solsolution for producing the MgF₂ optical thin film by the productionmethod of the present invention, wherein the sol solution contains MgF₂minute particles having an average particle diameter of 1 nm to not morethan 100 nm and one of silicon alkoxide and a polymer thereof. When thebinder-containing MgF₂ sol solution is coated and dried to produce SiO₂,it is possible to obtain a MgF₂ optical thin film in which the MgF₂minute particles are connected to one another by a small amount of SiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic magnified sectional view illustrating anoptical element according to a first embodiment.

FIG. 2 shows an optical system according to the first embodiment.

FIG. 3 shows an electron micrograph illustrating a surface and a crosssection of an MgF₂—SiO₂ optical thin film obtained by Example 1.

FIG. 4 shows an electron micrograph illustrating a surface and a crosssection of an MgF₂—SiO₂ optical thin film obtained by Example 7.

FIG. 5 shows a graph illustrating a spectral reflection characteristicof an optical element including an MgF₂—SiO₂ optical thin film providedon a resin layer manufactured in Example 29.

FIG. 6 shows a graph illustrating a spectral reflection characteristicof a multilayered optical thin film of Example 34.

FIG. 7 shows a graph illustrating a spectral reflection characteristicof a multilayered optical thin film of Example 35.

FIG. 8 shows a schematic sectional view illustrating an optical elementaccording to a second embodiment of the present invention.

FIG. 9 shows a schematic sectional view illustrating an optical elementof Example 36 of the present invention.

FIG. 10 shows a graph illustrating a relationship between wavelength andreflectance when a light comes into a multilayered antireflection filmconcerning Example 36 of the present invention.

FIG. 11 shows a graph illustrating the relationship between thewavelength and the reflectance when the light comes into themultilayered antireflection film concerning Example 36 of the presentinvention at angles of 30 degrees, 45 degrees, and 60 degrees.

FIG. 12 shows a graph corresponding to FIG. 10 concerning ComparativeExample 5 in comparison with Example 36 of the present invention.

FIG. 13 shows a graph corresponding to FIG. 11 concerning ComparativeExample 5 in comparison with Example 36.

FIG. 14 shows a graph corresponding to FIG. 10 in a case that asubstrate according to Example 37 of the present invention has arefractive index of 1.46.

FIG. 15 shows a graph corresponding to FIG. 10 in a case that thesubstrate according to Example 37 of the present invention has arefractive index of 1.62.

FIG. 16 shows a graph corresponding to FIG. 10 in a case that thesubstrate according to Example 37 of the present invention has arefractive index of 1.74 n.

FIG. 17 shows a graph corresponding to FIG. 10 in a case that thesubstrate according to Example 37 of the present invention has arefractive index of 1.85.

BEST MODE FOR CARRYING OUT THE INVENTION

First and second embodiments of the present invention will besuccessively explained below.

First Embodiment

FIG. 1 shows an optical element in which an MgF₂ optical thin film(MgF₂—SiO₂ film) of the first embodiment is formed. The optical element100 includes a base material 10 and the MgF₂ optical thin film 11 whichis stacked on a flat optical surface of the base material 10. The basematerial 10 is formed of, for example, glass, plastic or the like havinga refractive index of 1.4 to 2.1, and may be a plate member or a lens.The optical surface of the base material 10 may be formed to have acurved surface form.

The optical thin film 11 is an antireflection film which is stacked onat least one optical surface of the base material 10 onto which thelight is radiated. In this embodiment, the optical thin film 11 is asinglelayered MgF₂ antireflection film.

The optical thin film 11 includes MgF₂ minute particles 12 and anamorphous silicon oxide-based binder 13. The connection is made by theamorphous silicon oxide-based binder 13 between a large number of theMgF₂ minute particles 12 and between the large number of the MgF₂ minuteparticles 12 and the base material 10. In FIG. 1, the binder 13 isdepicted to surround the circumferences of the respective MgF₂ minuteparticles 12.

The MgF₂ minute particles 12 are minute particles composed of MgF₂crystals. It is appropriate that the MgF₂ minute particles 12 are highlycrystalline minute particles preferably having an average particlediameter of 1 nm to 100 nm, for the following reason. That is, thehighly crystalline MgF₂ minute particles 12 easily form a large numberof voids 14 between the large number of MgF₂ minute particles 12, andthus it is easy to suppress the change into any dense property whichwould be otherwise caused, for example, by the mutual adhesion oragglutination between the minute particles during the production.

The amorphous silicon oxide-based binder 13 is composed of an oxide suchas amorphous silica which is capable of forming the network structurecomposed of SiO₂. The amorphous silicon oxide-based binder 13 exists inirregular shapes between the large number of MgF₂ minute particles 12disposed mutually closely (in abutment) or between the base material 10and the MgF₂ minute particles 12 disposed closely or in abutment withrespect to the base material 10, to thereby make the integral connectionor bonding therebetween.

The amorphous silicon oxide-based binder 13 as described above can beused at an arbitrary ratio with respect to the MgF₂ minute particles 12.However, it is preferable to use the MgF₂ minute particles 12 in asmaller amount within a range in which the strength of the film itselfand the adhesive force with respect to the base material 10 aresufficiently obtained. It is appropriate that the amorphous siliconoxide-based binder 13 exists in an amount of 10% by weight to 30% byweight with respect to the MgF₂ minute particles 12, for the followingreason. That is, when the amorphous silicon oxide-based binder 13 existsat the rate as described above, then the MgF₂ minute particles 12 can beconnected to one another while suppressing the amount of use of theamorphous silicon oxide-based binder 13 which is somewhat inferior inthe environment resistance, and both of the film strength and theenvironment resistance can be achieved.

In the case of the MgF₂ optical thin film 11, the amorphous siliconoxide-based binder 13, which is arranged on the film surface to beirradiated with the light L, is formed to have a thin thickness. Theamorphous silicon oxide-based binder 13, which exists on the surfaces ofthe MgF₂ minute particles 12 arranged on the film surface, has thethickness which is not more than 5% of the wavelength of the light L tobe radiated. It is preferable that the amorphous silicon oxide-basedbinder 13, which exists between the MgF₂ minute particles 12 and betweenthe large number of MgF₂ minute particles 12 and the base material 10,has the thickness which is thinner than the particle diameter of theMgF₂ minute particles 12. The thickness of the amorphous siliconoxide-based binder 13 may be partially formed to be thicker than theparticle diameter, for any reason in view of, for example, theproduction.

When the thickness of the amorphous silicon oxide-based binder 13existing on the surfaces of the MgF₂ minute particles arranged on thefilm surface is thicker than 5% of the wavelength of the light L to beradiated, the film is regarded as an optically dense silica film. Theuppermost layer is the silica film having a refractive index of 1.42.When the thickness is thinner than 5% of the wavelength of the light Lto be radiated, the optical influence is negligible. Therefore, the MgF₂optical thin film is provided, in which the uppermost layer has a lowrefractive index. The thickness of the amorphous silicon oxide-basedbinder 13 can be measured by the measurement of the transmittance andreflection spectral characteristics or by the (scanning type) electronmicroscopic observation of the cross section of the film.

The MgF₂ optical thin film 11, for which the thickness of the binder isadjusted as described above, can have a refractive index of 1.10 to1.50. The film strength, which is measured by the microindentationmethod, can be made to be not less than 30 MPa and preferably not lessthan 110 MPa.

As for the MgF₂ optical thin film 11, the film strength may be alsoimproved such that the large number of voids 14, which are formedbetween the large number of MgF₂ minute particles 12, are filled withthe amorphous silicon oxide-based binder 13. However, the large numberof voids 14 can be maintained without being filled with the amorphoussilicon oxide-based binder 13. Accordingly, it is possible to reduce therefractive index of the MgF₂ optical thin film 11.

In this embodiment, the MgF₂ optical thin film 11 has a porous structurein which the large number of voids 14 are irregularly formed mutuallyamong the base material 10, the large number of MgF₂ minute particles12, and the amorphous silicon oxide-based binder 13 connecting them. Inthe porous structure, it is preferable that the percentage of voids orthe porosity is not more than 50%, for the following reason. That is, ifthe porosity is high, then the refractive index is lowered with ease,but the mechanical strength of the film becomes too low, and the film iseasily exfoliated, for example, by being manually wiped.

The MgF₂ optical thin film 11 as described above may be formed on theflat optical surface of the base material 10 as shown in FIG. 1.However, the MgF₂ optical thin film 11 may be formed on a curved opticalsurface. In this case, the MgF₂ optical thin film 11 can be also formedon a curved surface on which (effective lens diameter D)/(lens radius R)is 0.5 to 2 and especially 0.5 to 1. D/R indicates the degree of thecurved surface of the lens. D/R of 2 indicates a complete hemisphericallens. As the value is smaller, the curve of the lens becomes gentler.

When the radius of curvature of the base material 10 is small, and/orwhen the areal size is large, then the optical thin film cannot beformed to have any uniform thickness as a whole when the optical thinfilm is formed by the dry process such as the vacuum vapor depositionmethod, the sputtering method and the like. Usually, the thickness on asurface portion having an inclination with respect to a supply directionin which the raw material is supplied is thinner than the thickness ofthe film formed on a surface portion more perpendicular to the supplydirection than the inclined surface portion. However, the MgF₂ opticalthin film 11 of this embodiment is formed by the wet process asdescribed above. Therefore, the uniform thickness can be provided on theentire optical surface.

According to the MgF₂ optical thin film having the structure asdescribed above, the MgF₂ minute particles are used as the mainconstitutive substance of the film, and the connection is made betweenthe MgF₂ minute particles by the amorphous silicon oxide-based binder.Therefore, the strong connection can be made between the MgF₂ minuteparticles and between the MgF₂ minute particles 12 and the base material10. Accordingly, it is possible to improve the film strength and theadhesive force between the film and the base material. The MgF₂ minuteparticles, which are the main constitutive substance, are excellent inthe environment resistance. Further, the amorphous silicon oxide-basedbinder 13, which is composed of SiO₂ that is somewhat inferior in theenvironment resistance, merely makes the connection between the MgF₂minute particles 12 and between the MgF₂ minute particles 12 and thebase material 10. Therefore, it is possible to decrease the amount ofuse of the amorphous silicon oxide-based binder 13, and it is easy tosecure the environment resistance as the entire film. Further, therefractive index of the thin film can be reduced by using the MgF₂minute particles having the refractive index of 1.38. The refractiveindex of SiO₂ is relatively low, i.e., 1.42. Therefore, even when thecomposite is formed together with MgF₂, the refractive index of the filmis not raised so much.

Further, the amorphous silicon oxide-based binder 13, which exists onthe surfaces of the MgF₂ minute particles 12 arranged on the surface ofthe MgF₂ optical thin film 11, has the thickness which is not more than5% of the wavelength of the light L to be radiated. Therefore, anydensified, thick layer composed of the amorphous silicon oxide-basedbinder 13 is not formed on the film surface of the MgF₂ optical thinfilm 11, and it is possible to suppress the refractive index of the MgF₂optical thin film 11 to be low, which in turn makes it possible toobtain the excellent optical characteristics including, for example, thereflectance.

In particular, the MgF₂ optical thin film 11 as described above isformed on the outermost surface onto which the light L is radiated.Therefore, it is possible to obtain the sufficient antireflectionperformance by sufficiently lowering the refractive index of the MgF₂optical thin film 11.

Next, an explanation will be made with reference to FIG. 2 about animaging optical system provided with the MgF₂ optical thin films asdescribed above. The imaging optical system 118 includes a plurality ofoptical elements arranged between an object and an image plane, and isused as a zoom lens for a camera. The plurality of optical elementsinclude, in an order from the side of the object, a plane-parallel Fwhich is used as a protective glass, a negative meniscus lens L1 whichhas a convex surface directed toward the object, a cemented lens whichis obtained by sticking a negative meniscus lens L2 having a convexsurface directed toward the object and a negative meniscus lens L3having a convex surface directed toward the object, a double-concavelens L4, a double-convex lens L5, a cemented lens which is obtained bysticking a negative meniscus lens L6 having a convex surface directedtoward the object and a double-convex lens L7, an aperture diaphragm P,a cemented lens which is obtained by sticking a double-convex lens L8and a double-concave lens L9, a cemented lens which is obtained bysticking a negative meniscus lens L10 having a convex surface directedtoward the object and a double-convex lens L11, and a double-convex lens12. The plurality of optical elements are arranged so that an image ofthe object is formed on the image plane I.

The MgF₂ optical thin films are formed on one or both surfaces of a partor all of the plurality of optical elements.

The ghost, which is generated on a surface of the plane-parallel Fpositioned most closely to the side of the object in the imaging opticalsystem 118, the surface being on the side of the image, can beeffectively avoided by forming the MgF₂ optical thin film 11 on thissurface. The same or equivalent effect can be exhibited by the MgF₂optical thin film 11, even when the imaging optical system 118 is usedas an observation optical system in which an ocular lens is provided onthe side of the image plane of the imaging optical system describedabove. It is possible to observe a sharp image in which the ghost andthe flare are suppressed.

In the imaging optical system as described above, the MgF₂ optical thinfilm is provided on a surface of at least one of the optical elements.Therefore, it is possible to achieve the more excellent opticalperformance including, for example, the reflection characteristic with asmaller number of stacked layers.

This embodiment has been explained as illustrated by an example in whichthe singlelayered MgF₂ optical thin film is formed on the opticalsurface. However, it is also possible to form a multilayered opticalthin film on the optical surface. In this case, it is possible to usethe MgF₂ optical thin film for one layer among the multilayered opticalthin film.

When the low refractive index film using the MgF₂ optical thin film asdescribed above is used as the single layer so that the low refractiveindex film is used to form the multilayered film by making thecombination with the film formed by the dry process such as the vacuumvapor deposition method, the sputtering method, the CVD method and thelike, the MgF₂ optical thin film as described above, or the filmobtained by a known wet process, it is possible to exhibit the moreexcellent optical performance.

For example, the multilayered optical thin film, in which the lowrefractive index MgF₂ optical thin film having the refractive index ofnot more than 1.30 is arranged at the uppermost layer, makes it possibleto remarkably improve the wavelength band characteristic or the incidentangle characteristic, and makes it possible to suppress the reflectanceto be low with respect to the light allowed to come from a wide anglerange, and it is possible to suppress the reflectance to be low over awide wavelength region. In this case, as an underlying film, it ispossible to appropriately select and use, for example, a film based onthe dry process and a film based on the wet process having been hithertoused. When the MgF₂ optical thin film 11 formed by the wet process isused for the underlying film, it is easy to form all of the layersincluding the uppermost layer to have a uniform film thickness.

A multilayered optical thin film, which includes two layers of theadjacent MgF₂ optical thin films as described above, can be also formedon the optical surface onto which the light is to be radiated. In thiscase, the following film construction is appropriate. That is, a MgF₂optical thin film 11, in which the refractive index is as low aspossible, is arranged for the uppermost layer. However, another MgF₂optical thin film 11, in which the refractive index is relatively high,is included for the underlying film.

Further, the MgF₂ optical thin films 11 can be stacked adjacently. Inthis case, it is preferable that the difference in the refractive indexbetween the adjacent MgF₂ optical thin films is 0.02 to 0.23. It isappropriate that the refractive index of the MgF₂ optical thin filmdisposed on the inner side is higher than the refractive index of theMgF₂ optical thin film disposed on the outer side. Accordingly, it ispossible to improve, for example, the wavelength band characteristic inthe same manner as in a general antireflection film. Further, when therefractive index of the MgF₂ optical thin film disposed on the innerside is formed to be higher than the refractive index of the MgF₂optical thin film disposed on the outer side, it is possible tostrengthen the film strength of the optical thin film disposed on theinner side as compared with the film strength of the optical thin filmdisposed on the outer side. Therefore, it is easy to perform thestacking, and it is easy to perform the production.

Next, an explanation will be made about a method for producing the MgF₂optical thin film as described above. The MgF₂ optical thin film 11 asdescribed above is produced as follows. That is, a sol solution, inwhich the MgF₂ minute particles 12 having the average particle diameterof 1 nm to 100 nm are dispersed, is prepared, and a binder solution,which contains the component capable of forming the amorphous siliconoxide-based binder 13 by the reaction, is prepared. They are supplied tothe optical surface of the base material 10 so that a large number ofthe MgF₂ minute particles 12 are deposited. Further, the connection ismade with the amorphous silicon oxide-based binder 13 between the MgF₂minute particles 12 and between the MgF₂ minute particles 12 and thebase material 10.

The sol solution, in which the MgF₂ minute particles 12 are dispersed,can be prepared by mixing and reacting a magnesium compound and afluorine compound in the solvent to synthesize the MgF₂ minuteparticles.

Those usable as the magnesium compound include, for example, acetic acidsalt, chloride, alkoxide, and the like, and it is suitable to usemagnesium acetate. Those usable as the fluorine compound include, forexample, aqueous solution of hydrogen fluoride (hydrofluoric acid),anhydrous hydrogen fluoride, trifluoroacetic acid, and the like. It issuitable to use hydrofluoric acid. Those usable as the solvent includeorganic solvent such as alcohol. It is suitable to use methanol.

When the solvent such as methanol, which has the high velocity ofvaporization, is used, then the velocity of vaporization is quick duringthe film formation, and it is not easy to form the film having theuniform film thickness. Therefore, it is preferable to perform thesubstitution after the synthesis with any solvent having a lower vaporpressure such as higher alcohol including, for example, propanol,butanol and the like.

In this synthesis reaction, it is preferable to enhance thecrystallization property of the MgF₂ minute particles produced in thesolvent, for the following reason. That is, when the crystallizationproperty is enhanced, it is possible to suppress the densification whichwould be otherwise caused by the mutual adhesion or agglutination of theminute particles, even when the MgF₂ minute particles are accumulated ordeposited when the MgF₂ optical thin film is formed. Accordingly, thepores can be formed sufficiently to provide the porous property.

In order to enhance the crystallization property of the MgF₂ minuteparticles, it is preferable that the pressurizing treatment and/or theheat treatment is performed after mixing the magnesium compound and thefluorine compound. When the sol solution is subjected to, for example, atreatment at a high temperature and a high pressure, the crystallizationand the grain growth of the MgF₂ minute particles are caused. It ispossible to form the porous film having the higher porosity, i.e., thelow refractive index film. As described later on, when the strength ofthe porous film is raised, the refractive index is raised as well.Therefore, it is preferable that the sol solution, with which the MgF₂film having the sufficiently low refractive index can be obtained, isused as the base in order to obtain the film which has the lowrefractive index and the high strength.

When magnesium acetate is used as the magnesium compound, and methanolis used as the solvent, then acetic acid and methanol can be reactedwith each other to produce methyl acetate by performing the hightemperature high pressure treatment, which is especially preferred, forthe following reason. That is, when a large amount of acetic acid iscontained in the MgF₂ sol solution, the sol solution is geleted (gel isformed) when the sol solution is concentrated, which is difficult to besubjected to the coating. This makes it impossible to form any thickMgF₂ optical thin film in some cases.

The inventors have found out that the molar ratio of fluorine containedin the fluorine compound to magnesium contained in the magnesiumcompound as the raw material for preparing the MgF₂ sol solution(hereinafter referred to as “F/Mg ratio” in some cases) affects therefractive index of the MgF₂ optical thin film. In other words, evenwhen the MgF₂ films are treated with the SiO₂ solutions having a sameconcentration, the final refractive index differs among the MgF₂ filmsformed with the MgF₂ sol solutions having different ratios ofhydrofluoric acid/magnesium acetate.

Therefore, when the MgF₂ sol solution is prepared, it is preferable thatthe F/Mg ratio is within a predetermined range. It is preferable toadopt a range of 1.9 to 2.0. If the F/Mg ratio is too low, then theobtained film tends to be dense, and the refractive index tends toincrease. On the other hand, if the F/Mg ratio exceeds 2.0, the solsolution is easily geleted during the preparation of the sol solution.

When the F/Mg ratio is appropriately adjusted within the range of 1.9 to2.0, the refractive index of the obtained MgF₂ optical thin film can beadjusted to have the desired value.

When the F/Mg ratio is relatively high, i.e., 1.99 to 2.00, therefractive index is hardly increased, even when the concentration of thebinder solution described later on is made to be relatively high.Therefore, it is preferable to manufacture the low refractive indexfilm. On the other hand, when the F/Mg ratio is lowered to about 1.95,the refractive index is increased even when the binder solution havingthe relatively low concentration is used. Therefore, it is preferable tomanufacture the high refractive index film, probably for the followingreason. That is, it is considered that the surfaces of the MgF₂particles synthesized with the low F/Mg ratio are unstable, and thevoids 14, which exist between the minute particles, tend to be collapsedwhen the particles are accumulated to form the film. Therefore, it isconsidered that the refractive index is increased.

As described above, when the F/Mg ratio and the SiO₂ solutionconcentration are adjusted so that the refractive index is adjusted tohave the desired value, the refractive index can be optimally adjustedas the antireflection film for a variety of base materials havingdifferent refractive indexes. Therefore, it is possible to manufacturethe antireflection film having the excellent performance. It ispreferable that the MgF₂ concentration of the MgF₂ sol solution is lessthan 3%, for the following reason. That is, the refractive index can belowered as the concentration is higher, but the gelation is easilycaused when the concentration is too high.

Subsequently, the binder solution is prepared, which contains thecomponent capable of forming the amorphous silicon oxide-based binder bythe reaction.

The component, which is capable of forming the amorphous siliconoxide-based binder by the reaction, is a substance to be used to improvethe mechanical strength of the MgF₂ porous film and the adhesive forcewith respect to the base material. Substrates assumed include, forexample, a raw material substance which finally serves as thenetwork-forming oxide, and a precursor substance which is in the statebefore being converted into the network-forming oxide. Thenetwork-forming oxide includes, for example, the so-called glass-formingoxide, for which it is preferable to use a substance mainly composed ofSiO₂. As for the binder solution, it is preferable to use a solutionwhich produces SiO₂ by the heat treatment.

The representative substance of the organic silicon compound, whichproduces SiO₂ by the heat treatment, includes silicon alkoxide andperhydropolysilazane.

The alkoxysilane includes tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane, tetrabutoxysilane, tetratrimethoxysilane,methyltriethoxysilane, methyltripropoxysilane, methyltributoxysilane,ethyltrimethoxysilane, ethyltriethoxysilane, ethyltripropoxysilane,ethyltributoxysilane, propyltrimethoxysilane, propyltriethoxysilane,propyltripropoxysilane, propyltributoxysilane, dimethyldimethoxysilane,dimethyldiethoxysilane, dimethyldipropoxysilane, dimethyldibutoxysilane,diethyldimethoxysilane, diethyldiethoxysilane, diethyldipropoxysilane,diethyldibutoxysilane, methylethyldimethoxysilane, andmethylpropyldiethoxysilane.

When the alkoxysilane is used, it may be used as it is. However, whenthe hydrolysis and the condensation polymerization are previouslyperformed with an acid catalyst, the heat treatment temperature requiredfor the conversion into SiO₂ can be lowered, which is preferred.

The use of tetraethoxysilane as the alkoxysilane to form the networkstructure by performing the hydrolysis is described in “Science ofSol-Gel Method” written by Sumio Sakuhana and published by AGNE Shofusha(1989). According to this document, when a mixture solution oftetraethoxysilane, water, acid, and alcohol is agitated at a temperaturefrom room temperature to 80° C., the reaction is caused in accordancewith the following formula.

nSi(OC₂H₅)₄+4nH₂O→nSi(OH)₄+4nC₂H₅OH   (1)

When the solution containing produced Si(OH)₄ is polymerized byperforming the heat treatment, SiO₂ solid is obtained.

When polysilazane is used as the organic silicon compound, the reactionis caused with the water content in the air to form SiO₂ in accordancewith the following reaction formula.

SiH₂NH+2H₂O→SiO₂+NH₃+2H₂   (2)

When a polysilazane solution, to which a small amount of the amine-basedcatalyst is added, is used, the reaction is advanced even at roomtemperature to successfully cause the conversion into SiO₂. Usually, theconversion is caused into SiO₂ by performing the heat treatment at atemperature of not less than 50° C. at a high humidity in which therelative humidity is not less than 50%. In this procedure, thesufficient film strength is obtained in a relatively short period oftime, which is preferred.

The inventors have found out that, when the film is formed by using thebinder solution as described above, the concentration of the bindersolution is an important condition which affects the mechanical strengthand the, refractive index of the MgF₂ optical thin film. There is such atendency that the strength of the film is increased but the refractiveindex is also increased when the concentration of the binder solution isincreased. As the concentration is higher, the coefficient ofcontraction becomes higher as well, probably for the following reason.That is, it is considered that the strong force of contraction isallowed to act due to the change of the binder solution into SiO₂ as aresult of the heat treatment.

If the concentration of the binder solution is too high, a film of thebinder is consequently formed on the MgF₂ optical thin film. In thissituation, the SiO₂ film, which is dense and which has the relativelyhigh refractive index, is formed on the outermost surface, which isunfavorable to realize the wide band or the wide angle band of theoptical thin film.

Therefore, the concentration of silicon contained in the binder solutionis preferably not more than 5% by weight and especially favorably notmore than 2% by weight as represented by the SiO₂-convertedconcentration. In the case of the single coating method described lateron, the base material is coated with the coating liquid obtained bymixing the sol solution and the binder solution. However, even in thiscase, the concentration of silicon contained in the coating liquid ispreferably not more than 5% by weight and especially favorably not morethan 2% by weight as represented by the SiO₂-converted concentration.

The refractive index of the obtained MgF₂ optical thin film can beadjusted to have the desired value by adjusting the concentration of thebinder solution. The refractive index of the obtained MgF₂ optical thinfilm is lowered as the concentration of the binder solution is lowered.When no binder solution is used at all, the refractive index of the MgF₂optical thin film is lowered to 1.19. On the other hand, the refractiveindex is raised as the concentration of the binder solution is raised.The refractive index is raised to 1.42 when the SiO₂ film is formed withonly the binder solution. Therefore, the obtained MgF₂ optical thin filmcan be set to have any arbitrary refractive index within a range of 1.19to 1.42 by adjusting the concentration of the binder solution.

In the method of the present invention, the sol solution and the bindersolution, which are prepared as described above, are supplied onto thebase material 10 to perform the heat treatment. Accordingly, the largenumber of MgF₂ minute particles 12 are arranged on the surface of thebase material 10. Further, the connection is made with the amorphoussilicon oxide-based binder between the MgF₂ minute particles 12 andbetween the MgF₂ minute particles 12 and the base material 10.

In this procedure, the ratio of use between the MgF₂ sol solution andthe binder solution can be appropriately selected, for example, becausethe refractive index and the film strength of the obtained MgF₂ opticalthin film depend on the concentration. However, it is preferable thatthe ratio of use is to such an extent that the film strength, at whichthe manual wiping can be performed, is obtained, or that any thick SiO₂film is not formed on the outermost surface.

The following two methods are conceivable to supply the sol solution andthe binder solution to the surface which forms the optical surface ofthe base material 10.

The first method is a method (referred to as “double coating method”) inwhich the porous MgF₂ film is formed on the base material 10, and thenthe binder solution is coated and permeated thereto. The second methodis a method (referred to as “single coating method”) in which the bindersolution is previously mixed with the MgF₂ sol solution, and then themixture is coated onto the base material 10.

Among the two methods, in the case of the double (twice) coating method,any one of alkoxysilane and polysilazane can be used for the solsolution. However, in the case of the single (once) coating method,alkoxysilane is preferred, for the following reason. That is,polysilazane cannot be mixed with any solvent which contains water evenin any minute amount.

When polysilazane is used for the single coating method, it is necessarythat the solvent of the MgF₂ sol solution is subjected to the solventsubstitution with a non-aqueous solvent such as xylene and dibutyl etherother than alcohols, ketones, and esters, because the solvent of theMgF₂ sol solution is methanol.

In the case of the double coating method, the film is apparently driedimmediately after the coating with the MgF₂ sol solution. However, thesolvent still remains in the voids of the film, and hence the bindersolution is not sufficiently permeated. Accordingly, it is preferablethat by the double coating method, the binder solution is applied aftersufficiently drying the MgF₂ film formed by applying the sol solution.The MgF₂ film can be dried by being heated or by being depressurized.

If the binder solution is coated without sufficiently drying the coatingfilm after the MgF₂ sol solution has been coated, the binder film isconsequently formed on the MgF₂ film in the same manner as in the casein which the binder solution having the high concentration is used. Insuch a situation, the binder film, which is dense and which has therelatively high refractive index, is consequently formed on theoutermost surface, which is not preferred to realize the wide band orthe wide angle band of the optical thin film. Therefore, in order toprevent the binder film from being formed on the MgF₂ film, it ispreferable that the concentration of the binder solution is relativelylow, and the MgF₂ film is sufficiently dried.

In the single coating method, even when the concentration of the bindersolution is increased, the SiO₂ film is hardly produced on the MgF₂film. In addition, there is no need to perform the drying after formingthe MgF₂ film. Further, it is enough to perform the coating step once,which is efficient.

In the double coating method, although it is necessary to performcoating twice, namely one for the MgF₂ sol solution and the other forthe SiO₂ solution, this method is effective when the SiO₂ solutioncannot be mixed with the MgF₂ sol solution. In the case of the singlecoating method, it is enough to perform the coating step once, whichdecreases the cost. However, the single coating method is limited to thecase in which the SiO₂ solution can be mixed with the MgF₂ sol solution.

In the coating methods as described above, when the base material iscoated with the MgF₂ sol solution, the binder solution, or the coatingliquid, the coating can be performed by the spin coat method or the dipcoat method. When the coating is performed by the spin coat method,uneven film thickness and radial stripe may cause the problem. As forthe uneven film thickness, the coating can be performed uniformly bysubstituting a part of methanol as the solvent of the MgF₂ sol solutionwith the higher alcohol such as propanol, butanol, and pentanol asdescribed above. As for the radial stripe, the situation is alsoimproved by this method. However, it has been found out that the radialstripe can be greatly reduced when the spin coat is performed in anenvironment in which the relative humidity is not more than 40% andpreferably not more than 33%. The radial stripe is a stripe generatedradially from the center of the substrate to the circumference, which isthe phenomenon inherent in the spin coat. A wave is generated on theliquid film due to the friction between the air and the liquid filmdisposed on the substrate during the rotation, and the wave remains asit is to form the minute unevenness in the film thickness, which is themechanism for generating the radial stripes. A known effective way toreduce the radial stripes is to perform the spin coat whilesimultaneously rotating a disk disposed opposite to the substrate,thereby reducing the friction with the air. However, it is difficult todrip the coating liquid because the opposing disk becomes an obstacle tothe coating liquid, which is disadvantageous and thus not suitable forthe mass production.

In the present invention, this problem can be solved by merely loweringthe relative humidity. Therefore, it is unnecessary to change the stepsof the spin coat, and the productivity is not lowered. It is consideredthat when the low humidity is provided, the surface of the liquid filmis quickly dried, and hence the wave is scarcely caused to therebysuppress the generation of the radial stripe.

If the spin coat is performed in an environment in which the relativehumidity is less than 5%, then it is necessary to provide an extremelyexpensive and special dehumidifying apparatus, and the cost isconsequently increased. Therefore, it is preferable that the relativehumidity is not less than 5%.

When the coating is performed by the spin coat method, it is preferablethat the base material is rotated at a maximum number of revolutions of500 rpm to 9,000 rpm within 0 second to 3 seconds after supplying thecoating liquid or the sol solution to the base material. Accordingly, itis possible to suppress the unevenness of the film thickness and thegeneration of the radial stripe.

Subsequently, the heat treatment is performed after forming the film bycoating the MgF₂ sol solution and the binder solution on the basematerial as described above. When the heat treatment is performed, thenSiO₂ is produced from the binder solution allowed to exist between theMgF₂ minute particles and between the MgF₂ minute particles and the basematerial, and thus the mechanical strength of the film and the adhesiveforce with respect to the base material are greatly improved.

It is enough that the heat treatment temperature is a relatively lowtemperature of not less than about 50° C., when alkoxysilane ispreviously polymerized to provide a semi-processed product. Whenalkoxysilane is used as it is, a high temperature of not less than about300° C. is required.

When the temperature of the heat treatment becomes high, the basematerial is harmfully affected thereby as well. Therefore, it ispreferable that the heat treatment is performed at a lower temperaturedepending on the base material to be used. When the binder solutioncontaining alkoxysilane is used, then the temperature is preferably 50°C. to 300° C., for example, in the case of a glass base material, andthe temperature is preferably 30° C. to 150° C. in the case of a plasticbase material.

On the other hand, some types of polysilazane are converted into SiO₂ atroom temperature. However, in general, polysilazane is converted intoSiO₂ by the heat treatment performed at a temperature of not less than50° C. When the binder solution containing polysilazane is used, thenthe temperature is preferably 50° C. to 200° C., for example, in thecase of the glass base material, and the temperature is preferably 30°C. to 100° C. in the case of the plastic base material.

When the binder solution containing polysilazane is used, the SiO₂ filmis more densified as the humidity is more raised, which is preferred.

According to the production method as described above, it is possible toform the MgF₂ optical thin film (MgF₂—SiO₂ film) which can be wipedmanually and which has the refractive index lowered to 1.23. Further,the film can be formed to have the uniform thickness even on the lenshaving the small radius of curvature on which the film has beendifficult to be formed by the conventional dry process. As for theantireflection film, the satisfactory antireflection effect is exhibitedfrom the ultraviolet region to the near infrared region. Further, it ispossible to control the refractive index of the film. Therefore, whenthe optimum refractive index of the film, which is adapted to therefractive index of the substrate, is selected, it is possible toprovide the excellent antireflection film even in the case of the singlelayer.

The heat treatment for the film can be performed even at the lowtemperature. Therefore, the antireflection film can be formed not onlyon the glass substrate but also on the cemented lens including theplastic substrate and the resin layer. Hence, the range of applicationof the antireflection film is extremely wide. The antireflection filmcan be also used for the precision optical instrument including, forexample, the camera lens, the microscopic objective lens, the binocularlens, the projector projection lens, the glass or plastic spectaclelens; as well as for the display including, for example, the liquidcrystal display device, the plasma display, the electroluminescencedisplay, and the cathode ray tube display device; the window glass andthe show window; and the like.

Examples of the first embodiment will be explained below.

Preparation of MgF₂ Sol Solution

MgF₂ sol solutions were prepared as follows by using hydrogen fluoride(hydrofluoric acid) and magnesium acetate as row materials. Ahydrofluoric acid methanol solution was prepared, in which 50%hydrofluoric acid was dissolved in methanol. A magnesium acetatemethanol solution was prepared, in which magnesium acetate tetrahydratewas dissolved in methanol. A predetermined amount of the hydrofluoricacid methanol solution was added dropwise (by dripping) while agitatingthe magnesium acetate methanol solution to prepare an MgF₂ sol solution.In Examples described below, the mol ratio (F/Mg ratio) betweenhydrofluoric acid and magnesium acetate as the raw materials was changedwithin a range of 1.90 to 2.0 when the MgF₂ sol solutions were prepared.The concentration of MgF₂ contained in the MgF₂ sol solutions waschanged within a range of 0.5 to 2% by weight by adjusting theconcentrations of hydrofluoric acid and magnesium acetate.

The MgF₂ minute particles were immediately synthesized in the solsolution. However, even when the temperature was retained at roomtemperature as it was, the reaction was not completed. Therefore, thesynthesis reaction to produce MgF₂ was completed by performing atreatment at a high temperature and a high pressure. Simultaneously, thecrystallization and the grain growth were performed for the MgF₂ minuteparticles. The high temperature high pressure treatment was performed byplacing the MgF₂ sol solution in an autoclave vessel made of Teflon(trade name), which was then tightly closed, and by heating the MgF₂ solsolution in the vessel to 140° C. together with the vessel. The MgF₂minute particles of the sol solution have an average particle diameterof 1 nm to 100 nm (average particle diameter was 20 nm in Examplesdescribed below).

As a result of the high temperature high pressure treatment, acetic acidas a byproduct was reacted with methanol as the solvent to producemethyl acetate. A large amount of acetic acid is contained in the MgF₂sol solution. If the sol solution is concentrated without performing thehigh temperature high pressure treatment, then the sol solution isgeleted, which cannot be subjected to the coating. The sol solution wassuccessfully concentrated to have a high concentration without causingthe gelation by changing a greater part of acetic acid into methylacetate by the high temperature high pressure treatment. As a result, athick MgF₂ film of 1,000 angstroms to 5,000 angstroms, which wasdirected to the region ranging from the visible region to the nearinfrared region, was successfully manufactured.

Preparation of MgF₂ Sol Substitution Solution

The dispersion medium of the MgF₂ sol solution, subjected to the hightemperature high pressure treatment, was mainly composed of methanol.Therefore, even when the sol solution was subjected to the coating as itwas, the film was hardly formed uniformly, because the velocity at whichmethanol was vaporized was too quick. Therefore, the sol solution wasconcentrated by using a rotary evaporator, followed by being dilutedwith an organic solvent having a lower vapor pressure, including, forexample, higher alcohol such as propanol and butanol. Accordingly, apart of the methanol dispersion medium was substituted with such anorganic solvent to prepare an MgF₂ sol substitution solution.

The velocity of vaporization was suppressed in the MgF₂ sol substitutionsolution, and the coating was successfully performed to provide theuniform thickness. In particular, when an amount of the methanoldispersion medium, which was not less than the half of the total contentthereof, was substituted, the coating was successfully performeduniformly with ease without any unevenness. Further, in the case of theMgF₂ sol substitution solution, the refractive index and the thicknessof the obtained film were not changed at all even after the elapse of 6hours after the substitution. The film was formed satisfactorily.

Preparation of Binder Solution

A variety of binder solutions were prepared as the amorphous siliconoxide-based binders. A binder solution containing silicon alkoxide wasused, which was obtained by dissolving tetraethoxysilane (TEOS) inmethanol, and adding hydrochloric acid as a catalyst, followed by beingrefluxed at 80° C. for 24 hours. Another binder solution containingsilicon alkoxide was used, which was SUMICEFINE G-200B (trade name,SiO₂-converted concentration: 1.63% by weight) produced by SumitomoOsaka Cement Co., Ltd.

A binder solution containing polysilazane was used, which wasperhydro-polysilazane xylene solution NP110 (trade name, concentration:1% by weight) produced by Clariant Japan.

Preparation of Coating Liquid

When the single coating method is adopted, the binder solution is mixedwith the MgF₂ sol solution to prepare the coating liquid as follows. Abinder solution containing silicon alkoxide was added by 50% by weightat the maximum to a sol solution subjected to a high temperature highpressure treatment, and the mixture solution was concentrated by using arotary evaporator, followed by being diluted with an organic solventhaving a lower vapor pressure, including, for example, higher alcoholsuch as propanol and butanol. Accordingly, a part of the methanoldispersion medium was substituted with the organic solvent to preparethe coating liquid. The velocity of vaporization was suppressed for thecoating liquid, and the coating was successfully performed to providethe uniform thickness. In particular, when an amount of the methanoldispersion medium, which was not less than the half of the total contentthereof, was substituted, the coating was successfully performeduniformly with ease without any unevenness.

Evaluation of Refractive Index of Film

The reflection and transmission profiles of the substrate having thefilm formed on one surface were measured by using U-4000 produced byHitachi and CARY5 produced by Varian respectively. The film thicknessand the refractive index of the film at a wavelength of 550 nm weredetermined by calculation from obtained results.

Evaluation of Coefficient of Contraction

In the case of the single coating method, the coefficient of contractionof the film was determined by measuring the difference in the filmthickness generated by the presence and absence of the binder solution.In the case of the double coating method, the coefficient of contractionof the film was determined by measuring the difference in the filmthickness generated before and after the binder treatment.

Evaluation of Manual Wiping Resistance

CK wiper produced by Kanebo, which was a synthetic fiber wiping cloth,was used. The cloth was wetted with methanol, and the substrate havingthe film formed thereon was manually wiped with the cloth. It wasobserved whether or not any scratch appeared on the film by the manualwiping while illuminating the substrate from the back with a focusinglamp to evaluate the manual wiping resistance.

Method for Measuring Adhesive Force of Film

The adhesive force (film strength (MPa)) of the film with respect to thesubstrate was measured by using a thin film material evaluation system(MH-4000) produced by NEC-Sanei by the microindentation test method inaccordance with JIS R3255. The measurement was performed such that thesample was inclined by 35°, and the indentation velocity of the rubyindenter was 1.4 nm/sec.

EXAMPLES 1 TO 5

The hydrofluoric acid methanol solution was mixed with the magnesiumacetate methanol solution so that the MgF₂ concentration was 1%, andthat the hydrofluoric acid/magnesium acetate ratio was 1.95 to preparean MgF₂ sol solution. Subsequently, the sol solution was subjected tothe high temperature high pressure treatment at 140° C. for 24 hours.The average particle diameter of MgF₂ minute particles contained in thetreated MgF₂ sol solution was measured by the electron microscopicobservation, which was 20 nm. The sol solution was concentrated by usingthe rotary evaporator, followed by being diluted with 1-propanol tosubstitute 67% of the methanol solvent. The MgF₂ sol solution wasallowed to have an MgF₂ concentration of 2.5%, with which a silica glasssubstrate having a thickness of 3 mm was spin-coated at 2,000 rpm toform a porous MgF₂ film.

The substrate was dried at 70° C. for 1 hour, and then the temperaturewas returned to room temperature. An undiluted solution of SUMICEFINEG-200B or solutions obtained by diluting the undiluted solution 1.5times, 2 times, 2.5 times, and 3 times with 2-butanol were used asbinder solutions. The dried substrate was spin-coated with each of thebinder solutions at 2,000 rpm. The silicon concentration of theundiluted solution of SUMICEFINE G-200B is 1.63% by weight as convertedinto the concentration of SiO₂. The concentration of the 3-fold dilutedsolution is 0.54% by weight, the concentration of the 2.5-fold dilutedsolution is 0.65% by weight, the concentration of the 2-fold dilutedsolution is 0.82% by weight, and the concentration of the 1.5-folddiluted solution is 1.09% by weight. The SiO₂ solution was permeatedinto the porous MgF₂ film, followed by being heated at 160° C. for 1hour. SiO₂ was formed in the gaps of the porous film to form anMgF₂—SiO₂ film as the MgF₂ optical thin film.

The refractive indexes (before and after the SiO₂ treatment), thecoefficient of contraction, and the film strength of the obtainedMgF₂—SiO₂ films were measured respectively. As a result, as shown inTable 1, the following fact was revealed. That is, there was such atendency that as the concentration of the binder solution was higher,the refractive index of the MgF₂—SiO₂ film became higher, and the filmstrength became higher as well. When the treatment was performed withthe binder solution, each of the films was contracted. However, therewas such a tendency that as the concentration of the binder solution washigher, coefficient of contraction became higher. It is considered thatwhen the binder solution permeated into the porous film is changed intoSiO₂ by the heat treatment, then the strong force of contraction isallowed to act, and the entire film is contracted. According to theresults as described above, the following fact has been revealed. Thatis, the minute structure of the film, i.e., the porosity can becontrolled by the concentration of the SiO₂ solution, and the refractiveindex of the film can be controlled.

The manual wiping resistance was tested for the MgF₂—SiO₂ film obtainedin each of Examples 1 to 5. As a result, no scratch appeared on each ofthe films by the manual wiping.

EXAMPLES 6 TO 8

MgF₂—SiO₂ films were formed by the double coating method in the samemanner as in Examples 1 to 5 except that MgF₂ sol solutions wereprepared by mixing the hydrofluoric acid methanol solution with themagnesium acetate methanol solution so that the MgF₂ concentration was1%, and the ratio of hydrofluoric acid/magnesium acetate was 1.98, 1.99,and 2.0. Characteristics of the obtained films were measured. Resultsare shown in Table 1.

When the results are compared with each other with respect to Example 1,the following fact is appreciated. That is, there is such a tendencythat as the ratio of hydrofluoric acid/magnesium acetate is higher, therefractive index of the MgF₂—SiO₂ film becomes lower. In order to makethe refractive index of the MgF₂—SiO₂ film as low as possible, it isappreciated that the ratio of hydrofluoric acid/magnesium acetate ispreferably allowed to approach 2.0. However, if the ratio ofhydrofluoric acid/magnesium acetate exceeds 2.0, then the gelation iscaused at the stage of preparation of the sol solution, or the gelationis caused during the concentrating operation. Therefore, it has beenrevealed that the ratio of hydrofluoric acid/magnesium acetate ispreferably not more than 2.0.

The refractive index of the porous MgF₂ film, which is obtained beforebeing treated with the binder solution, is 1.23 in Example 1, and therefractive index is 1.20 in Example 7, the difference therebetween being0.03. However, the refractive index of the film, which is obtained afterbeing treated with the binder solution, is 1.34 in Example 1, and therefractive index is 1.26 in Example 7, the difference therebetween beingincreased to 0.08. That is, the following tendency was observed. Evenwhen the treatment is performed with the binder solution having the sameconcentration, the film tends to be densified after the treatment withthe binder solution when the ratio of hydrofluoric acid/magnesiumacetate is lowered. However, the film is hardly densified, when theratio of hydrofluoric acid/magnesium acetate is raised.

FIGS. 3 and 4 show scanning type electron micrographs of cross sectionsof the MgF₂—SiO₂ films obtained in Examples 1 and 7 respectively. Asshown in FIG. 3, in the case of the film of Example 1, it is appreciatedthat the film is relatively densified, because the edge of the filmcross section is sharp. As shown in FIG. 4, in the case of the film ofExample 7, the edge of the film cross section has a large number ofconcave/convex portions (porous property), which makes the edge to beindefinite.

Concave/convex portions, which were formed by the MgF₂ minute particles,were observed on the surfaces of the both films. Any SiO₂ film was notformed on both the MgF₂ films. The minimum wavelength of the light whichmay be radiated onto the MgF₂—SiO₂ film is 190 nm. Any SiO₂, which wasnot less than 5% of the wavelength (i.e., not less than 9.5 nm), did notexist on the surfaces of the MgF₂ minute particles arranged on the filmsurface (outermost portion). The specified thickness of SiO₂ disposed onthe surfaces of the MgF₂ minute particles existing on the film surfacewas not more than 1 nm in average. Any SiO₂, which was not less than theparticle diameter, did not exist between the MgF₂ minute particles.Accordingly, it has been revealed that a small amount of SiO₂ is formedonly at the interior (inside) of the porous MgF₂ film to connect theMgF₂ minute particles. The film thickness of SiO₂ was measured by theelectron microscope observation.

Further, the percentage of voids or the porosity was measured with amercury porosimeter for the MgF₂—SiO₂ films. As a result, it wassuccessfully confirmed that the porosity was 15 to 35% (not more than50%).

EXAMPLES 9 AND 10

MgF₂—SiO₂ films were formed by the double coating method in the samemanner as in Examples 1 to 5 except that MgF₂ sol solutions wereprepared under conditions in which the MgF₂ concentration was 0.5 and2%, and the ratio of hydrofluoric acid/magnesium acetate was 1.99.Characteristics of the obtained films were measured. Results are shownin Table 1.

When the results are compared with each other with respect to Example 7,the following fact is appreciated. That is, there is such a tendencythat as the MgF₂ concentration of the sol solution is higher, therefractive index of the film treated with the binder solution becomeslower. In order to make the refractive index of the MgF₂—SiO₂ film aslow as possible, it is appreciated that it is appropriate to perform thepreparation while making the MgF₂ concentration to be 2%. The MgF₂ solsolution having the concentration of 2% somewhat caused the gelation tosome extent, and the viscosity was high. However, when the treatment wasperformed at a high temperature and a high pressure, then the geldisappeared, and the MgF₂ sol solution was changed into a sol solutionhaving a low viscosity. When the concentration was 3%, the gel did notdisappear even when the treatment was performed at a high temperatureand a high pressure. Therefore, it is preferable that the concentrationis less than 3%. Further, only a limited volume of the material can bedealt with in the high temperature high pressure treatment. Therefore,it is effective that the concentration of the sol solution is made ashigh as possible.

The refractive indexes of the films obtained in Examples 8 and 10 werelowered to 1.23. Further, no scratch was formed on the films even whenthe manual wiping was performed by using a synthetic fiber wiping clothwhich was CK wiper produced by Kanebo.

According to the results of Examples 1 to 10, it has been successfullyconfirmed that the refractive index of the film after being treated withthe binder solution is greatly affected by the condition including, forexample, the ratio of hydrofluoric acid/magnesium acetate, theconcentration of the binder solution, and the MgF₂ concentration.

COMPARATIVE EXAMPLES 1 TO 3

MgF₂ sol solutions were prepared under conditions in which the MgF₂concentration was 1%, and the ratio of hydrofluoric acid/magnesiumacetate was 1.90, 1.95, and 2.0. Porous MgF₂ films were formed in thesame manner as in Examples 1 to 5 by using the sol solutions, withoutperforming the treatment with the binder solution. Characteristics ofthe obtained films were measured. Results are shown in Table 1.

As shown in Table 1, the film strength of the porous film not treatedwith the binder solution was extremely low. Even when the treatment wasnot performed with the binder solution, there was such a tendency thatthe refractive index was lowered as the ratio of hydrofluoricacid/magnesium acetate was more increased, when the comparison was madebetween Comparative Examples 1 and 2. The manual wiping resistance wastested for the MgF₂—SiO₂ films obtained in Comparative Examples 1 to 3.As a result, the film was wiped out by the manual wiping in the case ofany one of the films.

COMPARATIVE EXAMPLE 4

SUMICEFINE G-200B was concentrated with a rotary evaporator so that theconcentration was 3.73%, followed by being used for spin-coating on aBSC7 glass substrate having a thickness of 10 mm at 2,000 rpm.Subsequently, the glass substrate was heat-treated at 160° C. to form anSiO₂ film on the glass substrate.

The refractive index of the SiO₂ film was 1.42. The SiO₂ film was dense,because the density was close to the theoretical density. The wavelengthλ_(RM), at which the reflectance was minimum, was about 550 nm. However,when the environment resistance test (durability test), in which thefilm was retained for 20 hours at 70° C. at a relative humidity of 80%,was performed, λ_(RM) was subjected to the wavelength shift to about 650nm. On the other hand, even when the environment resistance test wasperformed for the film obtained in Example 1 in the same manner asdescribed above, then the reflectance was not changed, and only λ_(RM)was subjected to the wavelength shift by about 5 nm.

As described above, the film composed of only SiO₂ had a low environmentresistance, although the film was dense. Therefore, such a film was notsuitable for the optical thin film for the precision optical instrumentsuch as the camera and the microscope. On the other hand, in the case ofthe optical thin films, of Examples 1 to 7, which include MgF₂ as thebasic substance thereof it is appreciated that the optical performancecan be maintained over a long period of time, because these films havethe high environment resistance. Table 1 also shows characteristicsmeasured for the film obtained on Comparative Example 4.

TABLE 1 Mol ratio MgF₂ Film Concentration of hydro- concen- thick- offluoric tration ness SUMICEFINE Examples acid/ of of G-200B and magne-undiluted Refractive MgF₂ (wt. %, Comparative sium solution index offilm converted Examples acetate (wt. %) MgF₂ film (nm) into SiO₂) Ex. 11.95 1 1.23 890 0.54 Ex. 2 1.95 1 1.23 890 0.65 Ex. 3 1.95 1 1.23 8900.82 Ex. 4 1.95 1 1.23 890 1.09 Ex. 5 1.95 1 1.23 890 1.63 Ex. 6 1.98 11.21 970 0.54 Ex. 7 1.99 1 1.20 1100 0.54 Ex. 8 2.0 1 1.20 1270 0.54 Ex.9 1.99 0.5 1.25 1070 0.54 Ex. 10 1.99 2 1.20 1090 0.54 Comp. Ex. 1 1.901 1.25 830 — Comp. Ex. 2 1.95 1 1.23 890 — Comp. Ex. 3 2.0 1 1.20 1270 —Comp. Ex. 4 — — — — 3.73 Examples Refractive Coefficient and index Filmof Film Manual Comparative after SiO₂ thickness contraction strengthwiping Examples treatment (angstrom) (%) (MPa) resistance Ex. 1 1.34 80010 175 no scratch Ex. 2 1.36 820 8 188 no scratch Ex. 3 1.38 770 13 205no scratch Ex. 4 1.39 760 15 220 no scratch Ex. 5 1.41 600 33 237 noscratch Ex. 6 1.29 930 4 141 no scratch Ex. 7 1.26 1050 5 133 no scratchEx. 8 1.23 1220 4 125 no scratch Ex. 9 1.28 1020 5 145 no scratch Ex. 101.23 1050 4 130 no scratch Comp. Ex. 1 — — — 18 film wiped out Comp. Ex.2 — — — 22 film wiped out Comp. Ex. 3 — — — 19 film wiped out Comp. Ex.4 1.42 — 233 no scratch

EXAMPLES 11 TO 18

In Examples 11 to 18, MgF₂—SiO₂ films are formed by the single coatingmethod. The hydrofluoric acid methanol solution was mixed with themagnesium acetate methanol solution to prepare MgF₂ sol solutions sothat the MgF₂ concentration was 1%, and the ratio of hydrofluoricacid/magnesium acetate was 1.99 and 1.95. Subsequently, each of the solsolutions was subjected to a high temperature high pressure treatment at140° C. for 24 hours. The average particle diameter of MgF₂ minuteparticles contained in each of the treated MgF₂ sol solutions wasmeasured by the electron microscope observation, and the averageparticle diameter was 20 nm.

When SUMICEFINE G-200B as the binder solution was added by 10 to 50% byweight to each of the sol solutions, the solutions were successfullymixed uniformly. After that, each of the mixture solutions wasconcentrated by using a rotary evaporator, which was thereafter dilutedwith 1-propanol to substitute the methanol solvent to prepare the MgF₂coating liquid containing the binder having the silicon concentration of2.5 as converted into SiO₂.

The coating liquid was spin-coated on a silica glass substrate at 2,000rpm in the same manner as in Examples 1 to 5, and then the heattreatment was performed at 160° C. Thus, the MgF₂—SiO₂ film was formed.Characteristics of the obtained films were measured. Results are shownin Table 2.

According to the results of Examples 11 to 14 and Examples 15 to 18,there was such a tendency that the refractive index was raised inproportion to the amount of addition of SUMICEFINE G-200B in the bothcases in which the ratio of hydrofluoric acid/magnesium acetate were1.99 and 1.95 respectively. However, even when the amount of additionwas identical, the refractive index, which was obtained in a case thatthe ratio of hydrofluoric acid/magnesium acetate was 1.99, was lowerthan the refractive index which was obtained in a case that the ratiowas 1.95. When the refractive index was not less than 1.23, then noscratch was formed by the manual wiping, and the wiping resistance wassatisfactory.

The single coating method can be used provided that the binder solutioncan be mixed with the MgF₂ sol solution as in Examples 11 to 18. In thiscase, it is enough that the spin coat is performed only once. Therefore,the film can be formed efficiently as compared with the double coatingmethod. Further, SUMICEFINE G-200B is cured when the heating isperformed to at least a temperature of not less than about 50° C.Therefore, it is possible to improve the strength of the MgF₂ film, andit is possible to perform the manual wiping. When the films obtained inExamples 12 to 18 were heat-treated for 10 hours in the atmospheric airat 50° C., no scratch was formed even when the film was wiped with CKwiper in the same manner as in the treatment at 160° C. In the case ofthe lens in which the base material of the lens includes any resin orany resin layer, it is necessary that the heat treatment is performed ata temperature of not more than about 80° C. in order to avoid thedeformation of the resin. However, the low refractive index optical thinfilm, which had the film strength capable of performing the manualwiping, was successfully formed even on the lens as described above.

TABLE 2 Mol ratio of Amount of addition Concentration of hydrofluoricacid/ of SUMICEFINE G-200B SUMICEFINE in coating Refractive index Filmmagnesium acetate of (wt. %, ratio liquid (wt. %, converted after SiO₂strength Wiping Examples undiluted solution against base) into SiO₂)treatment (MPa) resistance 11 1.99 10 0.41 1.20 110 slightly scratched12 1.99 20 0.75 1.23 122 no scratch 13 1.99 30 1.03 1.26 131 no scratch14 1.99 50 1.49 1.33 170 no scratch 15 1.95 10 0.41 1.35 180 no scratch16 1.95 20 0.75 1.36 182 no scratch 17 1.95 30 1.03 1.38 195 no scratch18 1.95 50 1.49 1.40 213 no scratch

EXAMPLES 19 TO 29

Sol solutions were prepared under a condition in which the MgF₂concentration was 1%, and the high temperature high pressure treatmentwas performed at 140° C. for 24 hours. Each of the sol solutions wasconcentrated by using a rotary evaporator, followed by being dilutedwith 1-propanol to substitute 67% of the methanol solvent. The MgF₂concentration of each of the sol solutions was 2.5%, and the solsolution was spin-coated on a silica glass substrate having a thicknessof 3 mm at 2,000 rpm to form a porous MgF₂ film.

The substrate was dried at 70° C. for 1 hour, and then the temperaturewas returned to room temperature. The dried substrate was spin-coatedwith a polysilazane xylene solution as the binder solution(perhydro-polysilazane xylene solution NP110, concentration: 1% byweight) at 2,000 rpm, and then the heat treatment was performed.

In Examples 19 to 25, the ratio of hydrofluoric acid/magnesium acetatewas 1.99. The polysilazane xylene solution of 1% was used. Additionally,solutions of 0.25, 0.33, and 0.5%, which were obtained by diluting thepolysilazane xylene solution with xylene, were also used.

The heat treatment was performed under a condition at 150° C. in theatmospheric air in Examples 19 to 22, a condition at 70° C. at ahumidity of 80% in Examples 23 and 24, and a condition at 50° C. at ahumidity of 80% in Example 25.

The tendency, in which the refractive index of the film was raised asthe polysilazane concentration was raised, was the same as that observedfor the cases of Examples 1 to 5 in which SUMICEFINE was used for theSiO₂ solution. In Examples 19 and 20, any scratch was formed when thefilms were wiped with CK wiper, because of the low film strength.However, no scratch was formed when the refractive index was not lessthan 1.23. In Examples 23 to 25, the refractive index was rather raised,and the film was more densified in spite of the heat treatment performedat the low temperature of 50 to 70° C. (humidity: 80%), as compared withthe case in which the heat treatment was performed at 150° C. in theatmospheric air. That is, when polysilazane is used as the bindersolution, the humidity, which is provided during the heat treatment, isincreased as compared with the atmospheric air. Accordingly, SiO₂ isformed at the low temperature of 50 to 70° C., the porous MgF₂ film isstrengthened or reinforced, and the manual wiping can be performed. InExamples 26 to 29, the ratio of hydrofluoric acid/magnesium acetate was1.95, and the heat treatment was performed at the humidity of 80% at150° C. and 70° C. in the atmospheric air. When the polysilazaneconcentration was identical, the identical refractive index was obtainedunder any one of the heat treatment conditions. When the ratio ofhydrofluoric acid/magnesium acetate is 1.95, the MgF₂ is easilydensified. Therefore, the refractive index was not affected even whenthe heat treatment condition was changed.

In the case of any lens in which the base material includes any resin orany resin layer, if the heat treatment is performed at a hightemperature of not less than 100° C., the resin is consequentlydeformed. Therefore, it is necessary that the heat treatment isperformed at a temperature of not more than about 80° C. In this method,it is enough that the heat treatment is performed at a temperature of 50to 70° C. Therefore, the film can be formed without deforming the lensas described above.

Subsequently, a resin layer, which was composed of anultraviolet-curable resin having a refractive index of 1.55 and mainlycomposed of urethane acrylate and methacrylate, was formed to have athickness of 0.5 mm on the glass substrate by effecting the radiationwith a high pressure mercury lamp. Further, the MgF₂—SiO₂ film having arefractive index of 1.26, which was obtained in Example 23 or 25, wasformed on the surface of the resin layer. The resin layer is used for anaspherical lens made of resin.

FIG. 5 shows a result of the measurement of the spectral reflectance ofMgF₂—SiO₂ formed on the resin layer in the same manner as in Example 23.The spectral reflectance was measured by using a spectral reflectancemeasuring instrument U-4000 produced by Hitachi.

The reflectance was successfully lowered to 0.15% at a wavelength of 500nm. The film was strongly adhered to the resin layer as well. No scratchwas formed on the film even when the wiping was performed with CK wiperin the same manner as in the case in which the substrate was made ofglass. The SiO₂ binder exhibited the effect to improve the adhesiveforce of the film with respect to the rein substrate as well. The heattreatment was low, i.e., 50 to 70° C. Therefore, the rein layer was notexfoliated from the substrate, and the rein layer was not deformed andclouded.

TABLE 3 Mol ratio of Heat treatment condition Concentration Refractiveindex Film hydrofluoric acid/ tem- of polysilazane after SiO₂ strengthWiping Examples magnesium acetate pperature humidity (wt. %) treatment(MPa) resistance 19 1.99 150° C. atmospheric 0.25 1.21 115 slightly airscratched 20 1.99 150° C. atmospheric 0.33 1.21 117 slightly airscratched 21 1.99 150° C. atmospheric 0.5 1.23 127 no scratch air 221.99 150° C. atmospheric 1 1.30 138 no scratch air 23 1.99  70° C. 80%0.5 1.26 145 no scratch 24 1.99  70° C. 80% 1 1.33 176 no scratch 251.99  50° C. 80% 0.5 1.26 138 no scratch 26 1.95 150° C. atmospheric 0.51.30 146 no scratch air 27 1.95 150° C. atmospheric 1 1.36 184 noscratch air 28 1.95  70° C. 80% 0.5 1.30 151 no scratch 29 1.95  70° C.80% 1 1.36 198 no scratch

EXAMPLE 30

The hydrofluoric acid methanol solution was mixed with the magnesiumacetate methanol solution to prepare an MgF₂ sol solution so that theMgF₂ concentration was 2%, and that the ratio of hydrofluoricacid/magnesium acetate was 1.99. Subsequently, the sol solution wassubjected to a high temperature high pressure treatment at 140° C. for24 hours. The sol solution was concentrated by using a rotaryevaporator, followed by being diluted with 1-propanol to substitute 67%of the methanol solvent. The MgF₂ concentration of the sol solution was4%, and the sol solution was spin-coated on a silica glass substratehaving a thickness of 3 mm at 1,000 rpm in an environment in which theroom temperature was 24° C. and the relative humidity was 33% to form anMgF₂ film. As a result, the film was successfully formed uniformlywithout any unevenness. The refractive index was 1.19, and the thicknesswas 2,210 angstroms.

When the spin coat was performed under the same condition in anenvironment in which the relative humidity was 42%, stripes were formedradially from the center to the circumference of the substrate.

The substrate was dried at 70° C., and the temperature was returned toroom temperature. The substrate was spin-coated at 2,000 rpm with thebinder solution in which SUMICEFINE G-200B was diluted three times with2-butanol. The MgF₂—SiO₂ film, which was heat-treated at 150° C., had arefractive index of 1.20 and a thickness of 2,200 angstroms. When theevaluation was made for the wiping resistance with CK wiper, no scratchwas formed on the film.

Even when the MgF₂ film, on which the radial stripes were formed, wassubjected to the SiO₂ treatment, the radial stripes did not disappear.

When a coating liquid, which was obtained by adding SUMICEFINE G-200B asthe binder solution to the sol solution, was used, no radial stripe wasformed at a relative humidity of 33% as well in the same manner asdescribed above. However, radial stripes were formed at a relativehumidity of 42%.

EXAMPLE 31

The hydrofluoric acid methanol solution was mixed with the magnesiumacetate methanol solution to prepare an MgF₂ sol solution so that theMgF₂ concentration was 1%, and the ratio of hydrofluoric acid/magnesiumacetate was 1.95. Subsequently, the sol solution was subjected to a hightemperature high pressure treatment at 140° C. for 24 hours. The solsolution was concentrated by using a rotary evaporator, followed bybeing diluted with 1-propanol to substitute 67% of the methanol solvent.The MgF₂ concentration of the binder solution was 3.5%, and the bindersolution was spin-coated on a silica glass substrate having a thicknessof 3 mm in an environment in which the room temperature was 24° C. andthe relative humidity was 33% to form an MgF₂ film.

The maximum number of revolutions was 2,000 rpm. When the number ofrevolutions arrived at 2,000 in 5 seconds, radial stripes were formed.However, when the number of revolutions arrived at 2,000 in 1 second, noradial stripe was formed.

Even when the relative humidity is lowered, it is not necessarilypossible to completely prevent all of the radiation stripes, dependingon the ratio of hydrofluoric acid/magnesium acetate. However, in suchsituations, the radiation stripe was allowed to successfully disappearby making the number of second or seconds, which was required untilarrival at the maximum number of revolutions of the spin coat, to bewithin a relatively short period of time.

EXAMPLE 32

Each of the MgF₂—SiO₂ films as obtained in Examples 1, 15, and 24 havinga refractive index of 1.33 to 1.35 respectively was formed on an opticalglass substrate having a refractive index nd=1.80 in place of the silicaglass substrate. When the spectral reflectance of the obtained substratewas measured, the minimum value of the reflectance was 0.1%.

The MgF₂—SiO₂ film of Example 1 was formed on a concave lens having arefractive index nd=1.80 and a radius of curvature of 20 mm (D/R=1.83).The lens is one of lenses for constructing a lens system of a singlelens reflex camera. When the film was formed, 67% of the methanolsolvent was substituted with 1-propanol to prepare a sol solution toperform the spin coat by rotating the lens so that the number ofrevolutions arrived at 2,000 rpm in 1 second. The film thickness wascontrolled by changing the concentration of the sol solution so that thereflectance of the lens on which the MgF₂—SiO₂ film was formed wasminimized at a wavelength of 600 nm.

It has been revealed that the film can be formed to have the uniformthickness on the surface having the small radius of curvature even inthe case of the spin coat, unlike the general vacuum vapor depositionmethod.

The obtained concave lens was set to the lens system of the single lensreflex camera. When this camera was used to take a photograph, theghost, which would be generated when a strong point light source such asthe sun light was positioned at the corner of the image plane, had thecolor changed from the orange to the blue. The ghost was successfullymade inconspicuous. The following fact has been revealed. That is, theMgF₂—SiO₂ film can be formed uniformly according to the method of thepresent invention on the surface of the lens group constructing the lensof the single lens reflex camera, although it is difficult to form anyfilm uniformly on the surface by the vacuum vapor deposition method.Further, when this camera is used to take a photograph, it is possibleto greatly reduce the ghost.

EXAMPLE 33

Each of the MgF₂—SiO₂ films as obtained in Examples 4, 5, and 18 havinga refractive index of 1.39 to 1.41, respectively, was formed on anoptical glass substrate having a refractive index nd=2.02 in place ofthe silica glass substrate. When the spectral reflectance of theobtained substrate was measured, the minimum value of the reflectancewas 0.1%.

The MgF₂—SiO₂ film of Example 4 was formed on a convex surface having arefractive index nd=2.02 and a radius of curvature of 3.5 mm (D/R=1.90).The lens is one of lenses constructing an objective lens system of amicroscope. When the film was formed, 67% of the methanol solvent wassubstituted with 2-propanol to prepare a sol solution, followed by beingsubjected to the spin coat by rotating the lens so that the number ofrevolutions arrived at 7,000 rpm in 1 second. The film thickness wascontrolled by changing the concentration of the sol solution so that thereflectance of the lens on which the MgF₂—SiO₂ film was formed wasminimized at a wavelength of 550 nm. It has been revealed that the filmcan be formed to have the approximately uniform thickness even in thecase of the convex lens which is close to the hemisphere.

The obtained convex lens was incorporated into a part of the objectivelens of the microscope. When the microscope was used to perform thefluorescent observation by using a laser as an exciting light source,the formation of interference fringes, caused by the laser beam in theobservation field, was successfully suppressed to the minimum. Thefollowing fact has been revealed. That is, the MgF₂—SiO₂ film can beformed uniformly according to the method of the present invention on thesurface of the lens included in the lens group constructing theobjective lens of the microscope, although it is difficult to form anyfilm uniformly on the surface by the vacuum vapor deposition method.Further, when the microscope having the lens is used to perform theobservation, then it is possible to greatly reduce the ghost, and it ispossible to obtain a high contrast image.

EXAMPLE 34

A five-layered antireflection film, in which the MgF₂—SiO₂ film havingan refractive index of 1.23 as obtained in each of Examples 8, 10, 12,and 21 was arranged at the uppermost layer, was formed on DF13 opticalglass (nd=1.74). That is, Al₂O₃, MgF₂, and ZrO₂ films formed by the dryprocess and the MgF₂—SiO₂ film formed by the wet process were stacked onthe DF13 optical glass.

The film construction, the film formation method, and the film thicknessare as shown below in Table 4.

TABLE 4 Film Constitutive Film formation thickness substance method (nm)Fifth layer MgF₂ wet process 121.3 Fourth layer ZrO₂ vapor deposition11.1 method Third layer Al₂O₃ vapor deposition 10 method Second layerMgF₂ vapor deposition 28.8 method First layer Al₂O₃ vapor deposition 10method Substrate DF13 optical glass

FIG. 6 shows a result of the measurement of the reflectance of thesubstrate having the antireflection film formed thereon as describedabove. The reflectance is not more than 0.5% in the entire visibleregion at wavelengths from 400 nm to 800 nm. It is appreciated that theband is wide and the reflection is low. According to this result, theantireflection film, which has not been conventionally provided, hasbeen successfully manufactured in accordance with the present invention.

EXAMPLE 35

A two-layered antireflection film, in which the MgF₂—SiO₂ film having anrefractive index of 1.38 as obtained in each of Examples 3 and 17 wasarranged as the first layer and the MgF₂—SiO₂ film having an refractiveindex of 1.23 as obtained in each of Examples 8, 10, 12, and 21 wasarranged as the second layer, was formed on BSC7 optical glass(nd=1.52). The multilayered antireflection film was successfully formeduniformly even on the lens having a small radius of curvature, becauseboth of the two layers were formed by the wet film formation method.

The film construction, the film formation method, and the film thicknessare as shown below in Table 5.

TABLE 5 Film Constitutive Film formation thickness substance method (nm)Second layer MgF₂—SiO₂ film wet process 104.6 First layer MgF₂—SiO₂ filmwet process 93.3 Substrate BSC7 optical glass

FIG. 7 shows a result of the measurement of the reflectance of thesubstrate having the antireflection film formed thereon as describedabove. The difference in the refractive index between the mutuallyadjoining MgF₂—SiO₂ films is 0.02 to 0.23. Therefore, the reflectance isnot more than about 1% in the entire wavelength region at wavelengthsfrom 350 nm to 1,100 nm. Accordingly, it is appreciated the reflectionis low in the wide band ranging from the ultraviolet to the nearinfrared. According to this result, the antireflection film, which hasnot been conventionally provided, has been successfully manufactured inaccordance with the present invention.

Second Embodiment

FIG. 8 shows a structure of an optical element 110 according to thesecond embodiment.

With reference to FIG. 8, the optical element 110 includes amultilayered antireflection film 112 which includes several, which isnot less than three, of several types of layers having differentrefractive indexes, the layers being disposed on a flat optical surfaceof a substrate 111. Specifically, the substrate 111 may be formed of,for example, glass or plastic, and may have a form of plate member orlens. The optical surface may be a curved surface.

The multilayered antireflection film 112 is designed so that theuppermost layer 113, which makes contact on the side of a medium, hasthe refractive index which is set to be not more than 1.30 at the designcenter wavelength λ₀; and that the layers 114 other than the uppermostlayer are constructed by stacking a layer which has the refractive indexof not less than 2 at the design center wavelength λ₀ and a layer whichhas the refractive index of 1.38 to 1.7 at the design center wavelengthλ₀.

In the multilayered antireflection film 112, the layer 115, which makescontact with the substrate 111, has the refractive index which is 1.38to 1.7 at the design center wavelength λ₀. Further, the second layer 116as counted from the side of the medium has the refractive index which isset to be not less than 2 at the design center wavelength λ.

Each of the films of the multilayered antireflection film 112 asdescribed above may be formed by any one of the methods selected fromthe wet process including, for example, the sol-gel method and the like,and from the dry process including the vacuum vapor deposition method,the sputtering method, the ion plating method, the CVD method, and thelike. The methods for forming the layers may be different from eachother or identical with each other.

The material for the substrate 111 of the optical element 110 of thepresent invention is not specifically limited provided that the materialis an optical base material, and is preferably applicable to the opticalelement 110 such as the lens, the prism, the filter and the like. Theoptical element 110 as described above improves the optical performanceof the optical system in which the optical element 110 is incorporated.Further, the optical element 110 improves the performance of the opticaldevice which is provided with the optical system.

The optical element 110 as described above can be incorporated into anyone of the optical elements L1 to L12 of the imaging optical system 118explained with reference to FIG. 2 in the first embodiment. That is, themultilayered antireflection film 112 is formed on one or both surfacesof a part or all of the plurality of optical elements L1 to L12. In thiscase, the antireflection films 112 are applied, for example, to thesurfaces X, Y to which the flat surface and/or the concave surface isdirected or opposite as viewed from the aperture diaphragm P of theoptical system.

In the case of the imaging optical system 118 as described above,Rn×Rm≦0.002% is satisfied in the entire visible region provided that Rnrepresents a reflectance of normal incidence on an n-th ghost-generatingsurface in the optical system, and Rm represents a reflectance of normalincidence on an m-th ghost-generating surface. The multilayeredantireflection film 112 is applied to at least one surface of the n-thand m-th ghost-generating surfaces. The imaging optical system 118 isused in a wavelength region ranging from 400 nm to 700 nm.

In the case of the imaging optical system 118 as described above, themultilayered antireflection film 112 is constructed as follows. That is,three or more of the several types of layers having different refractiveindexes are stacked. The uppermost layer 113, which makes contact on theside of the medium, has the refractive index which is not more than 1.30at the design center wavelength λ₀; and the layers 114 other than theuppermost layer are constructed by stacking a layer which has therefractive index of not less than 2 at the design center wavelength λ₀and a layer which has the refractive index of 1.38 to 1.7 at the designcenter wavelength λ₀. Therefore, the wavelength band characteristic andthe incident angle characteristic are remarkably improved, and thereflectance can be suppressed to be low with respect to the lightallowed to come in a wide angle range. Further, the reflectance can besuppressed to be low over a wide wavelength region.

The layer 115, which makes contact with the substrate 111, has therefractive index which is 1.38 to 1.7 at the design center wavelengthλ₀. Further, the second layer 116 as counted from the side of the mediumhas the refractive index which is not less than 2 at the design centerwavelength λ₀. Therefore, it is possible to further suppress thereflectance to be low.

The optical element 110 has the multilayered antireflection film 112provided on the substrate 111. Therefore, it is possible to obtain theoptical element 110 in which the reflectance can be suppressed to be lowwith respect to the light allowed to come in a wide angle range, and thereflectance can be suppressed to be low over a wide wavelength region.

Further, even when the substrate 111 has any curved surface, withoutbeing limited to only the plane-parallel, then the reflectance can besuppressed to be low with respect to the light allowed to come in a wideangle range, and the reflectance can be suppressed to be low over a widewavelength region. In this case, the ghost, which would be generated onthe surface on the image side of the plane-parallel F positioned on theside nearest to the object of the imaging optical system 118, can beavoided more effectively by forming the multilayered antireflection film112 on the surface. The more excellent optical performance including,for example, the reflection characteristic and the like can be achievedwith a smaller number of the arranged multilayered antireflection films112, because the multilayered antireflection film 112 is provided on thesurface on which the ghost would be otherwise generated.

The reflectance is suppressed to be low for the optical element 110provided with the multilayered antireflection film 112 of the presentinvention. Therefore, when the optical element 110 is adopted for atleast one of the plurality of optical elements L1 to L12 of the imagingoptical system 118, the imaging optical system 118 can form an image inwhich the ghost and the flare are suppressed.

Further, Rn×Rm≦0.002% is satisfied (in the entire visible region)provided that Rn represents a reflectance of normal incidence on an n-thghost-generating surface in the optical system, and Rm represents areflectance of normal incidence on an m-th ghost-generating surface.Therefore, the imaging optical system 118 can form an image in which theghost and the flare are further suppressed.

Furthermore, the multilayered antireflection film 112 is applied to atleast one surface of the n-th and m-th ghost-generating surfaces.Therefore, the imaging optical system 118 can form an image in which theghost and the flare are further suppressed.

If the reflectances Rn, Rm of the normal incidence on the n-th and m-thghost-generating surfaces is Rn×Rm>0.002% in the visible light region,there is such a possibility that the ghost and the flare may begenerated conspicuously, and any obtained image may be deteriorated inquality.

The multilayered antireflection film 112 is applied to the surface towhich the flat surface or the concave surface is opposite as viewed fromthe aperture diaphragm P of the optical system. Therefore, an image, inwhich the ghost and the flare are further suppressed, can be obtainedmore effectively with the optical system. In other words, if thereflection is caused on the surface to which the flat surface or theconcave surface is opposite as viewed from the aperture diaphragm P ofthe optical system, the image is affected more greatly as compared withany case in which the reflection is caused on any other surface.Therefore, when the multilayered antireflection film 112 is provided onthe surface to suppress the reflection, it is possible to obtain animage in which the ghost and the flare are further suppressed moreeffectively, as compared with any case in which the multilayeredantireflection film 112 is provided on any other surface.

Further, the reflectance can be further lowered for the optical systemwhich is used in the wavelength region from 400 nm to 700 nm. Even whenan observation optical system, which has an ocular lens provided on theside of the image plane of the imaging optical system 118, is providedand used, the multilayered antireflection film 112 can exhibit the sameor equivalent effect. Accordingly, it is possible to observe a sharpimage in which the ghost and the flare are suppressed.

Examples of the second embodiment will be explained below.

EXAMPLE 36

As shown in FIG. 9, a film of Example 36 is provided as a wide bandmultilayered antireflection film 112 formed of five layers, in which thelow reflectance is realized in the entire visible region. The film ofExample 36 is constructed as shown in Table 6.

TABLE 6 Refractive Optical film Substance index thickness Medium airFifth layer SiO₂ + MgF₂ 1.26 0.269λ₀ Fourth layer ZrO₂ + TiO₂ 2.120.043λ₀ Third layer Al₂O₃ 1.65 0.217λ₀ Second layer ZrO₂ + TiO₂ 2.120.066λ₀ First layer Al₂O₃ 1.65 0.290λ₀ Substrate BK7 1.52

In this case, the wavelength 550 nm is the design center wavelength λ₀,and the medium is the air. The substrate 111 is borosilicate crownoptical glass (BK7) having a refractive index of 1.52 at λ₀. The stackedstructure is designed optimally for the substrate 111. That is, themultilayered antireflection film 112 is composed of five layers. Thefirst layer 121 (layer 115 which makes contact with the substrate 111),which is most closely to the substrate 111, is formed of aluminum oxide(Al₂O₃). The first layer 121 is formed such that the refractive index is1.65 (refractive index of 1.38 to 1.7), and the optical film thicknessis 0.290λ₀.

The second layer 122 is formed of a mixed layer (ZrO₂+TiO₂) composed ofzirconium oxide and titanium oxide, and is formed such that therefractive index is 2.12 (refractive index of not less than 2), and theoptical film thickness is 0.066λ₀.

The third layer 123 is formed of aluminum oxide (Al₂O₃), and is formedsuch that the refractive index is 1.65 (refractive index of 1.38 to1.7), and the optical film thickness is 0.217λ₀.

The fourth layer 124 (second layer 116 as counted from the side of themedium) is formed of a mixed layer (ZrO₂+TiO₂) composed of zirconiumoxide and titanium oxide, and is formed such that the refractive indexis 2.12 (refractive index of not less than 2), and the optical filmthickness is 0.043λ₀.

The fifth layer 125 (uppermost layer 113 which makes contact on the sideof the medium) is formed of a mixed layer (SiO₂+MgF₃) composed of silicaand magnesium fluoride, and is formed such that the refractive index is1.26 (refractive index of not more than 1.30), and the optical filmthickness is 0.269λ₀.

As described above, the first and third layers 121, 123 are theintermediate refractive index layers (refractive indexes are not lessthan 1.38 and not more than 1.7), the second and fourth layers 122, 124are the high refractive index layers (refractive indexes are not lessthan 2), and the fifth layer 125 is the low refractive index layer(refractive index is not more than 1.30).

The multilayered antireflection film having the structure as describedabove has the following spectral reflectance characteristic as obtainedupon the normal incidence as shown in FIG. 10. That is, it isappreciated that the reflectance is suppressed to be not more than 0.2%over the entire wavelength region from about 420 nm to 720 nm. Further,FIG. 11 shows the spectral reflection characteristics obtained when theangle of incidence is 30 degrees, 45 degrees, and 60 degrees. Thespectral reflectance characteristic was measured in the secondembodiment by using a spectral reflectance measuring instrument U-4000produced by Hitachi.

COMPARATIVE EXAMPLE 5

Table 7 shows a construction of a multilayered wide band antireflectionfilm based on the conventional technique in which the same medium andthe same substrate were used, in comparison with Example 36.

TABLE 7 Refractive Optical film Substance index thickness Medium airSeventh layer MgF₂ 1.39 0.243λ₀ Sixth layer ZrO₂ + TiO₂ 2.12 0.119λ₀Fifth layer Al₂O₃ 1.65 0.057λ₀ Fourth layer ZrO₂ + TiO₂ 2.12 0.220λ₀Third layer Al₂O₃ 1.65 0.064λ₀ Second layer ZrO₂ + TiO₂ 2.12 0.057λ₀First layer Al₂O₃ 1.65 0.193λ₀ Substrate BK7 1.52

FIG. 12 shows a spectral reflection characteristic of this multilayeredantireflection film upon the normal incidence. Similarly, FIG. 13 showsspectral reflection characteristics at angles of incidence of 30degrees, 45 degrees, and 60 degrees at which the light is allowed tocome into the multilayered antireflection film.

When Example 36 is compared with Comparative Example 5, the reflectanceof the antireflection film of Example 36 is as follows. That is, thereflectance upon the normal incidence is reduced by not less than ½ atsome portions as compared with the conventional technique. The extremelysatisfactory antireflection performance is provided over the entireregion. It is clear that the antireflection performance, in which thereflectance is extremely lower than that of the antireflection film ofthe conventional technique, is obtained when the angle of incidence isfurther increased.

EXAMPLE 37

In Example 37, as shown in Table 8, multilayered antireflection films112, each of which is formed of seven layers, are provided. Theantireflection films 112 are applied to a plurality of substrates 111having different refractive indexes.

TABLE 8 Optical Optical Optical Optical Refrac- film film film film tivethick- thick- thick- thick- Substance index ness ness ness ness Mediumair 1 Seventh SiO₂ + 1.26 0.275λ₀ 0.268λ₀ 0.271λ₀ 0.269λ₀ layer MgF₂Sixth ZrO₂ + 2.12 0.045λ₀ 0.057λ₀ 0.054λ₀ 0.059λ₀ layer TiO₂ Fifth Al₂O₃1.65 0.212λ₀ 0.171λ₀ 0.178λ₀ 0.162λ₀ layer Fourth ZrO₂ + 2.12 0.077λ₀0.127λ₀ 0.13λ₀ 0.158λ₀ layer TiO₂ Third Al₂O₃ 1.65 0.288λ₀ 0.122λ₀0.107λ₀ 0.08λ₀ layer Second ZrO₂ + 2.12 0 0.059λ₀ 0.075λ₀ 0.105λ₀ layerTiO₂ First Al₂O₃ 1.65 0 0.257λ₀ 0.03λ₀ 0.03λ₀ layer Refractive 1.46 1.621.74 1.85 index of substrate Corre- FIG. 8 FIG. 9 FIG. 10 FIG. 11sponding Fig.

That is, the multilayered antireflection films 112 were designed forfour types of the substrates 111 having the refractive indexes of 1.46,1.62, 1.74, and 1.85 at a wavelength of 550 nm. The respective designedvalues are shown in Table 8.

FIG. 14 shows a spectral reflection characteristic for the substrate 111having the refractive index of 1.46. FIG. 15 shows a spectral reflectioncharacteristic for the substrate 111 having the refractive index of1.62. FIG. 16 shows a spectral reflection characteristic for thesubstrate 111 having the refractive index of 1.74. FIG. 17 shows aspectral reflection characteristic for the substrate 111 having therefractive index of 1.85. According to these drawings, it is appreciatedthat the reflectance is suppressed to be not more than about 0.2% overthe entire wavelength region from about 420 nm to about 720 nm.

As described above, even when the refractive index of the substrate 111differs, the satisfactory antireflection performance, in which thereflectance is low over the wide band or region, can be obtained withthe five to seven layers in total by optimizing the film thickness ofeach of the layers, without drastically altering the basic construction.

As described above, the multilayered antireflection films 112 obtainedin Examples 36 and 37 exhibit the low reflectance characteristic withrespect to the incoming light in the visible region and the allowancefor the wide angle characteristic.

INDUSTRIAL APPLICABILITY

The MgF₂ optical thin film of the present invention realizes the lowreflectance in the wide angle range in the visible light region (400 nmto 800 nm). When the optical element, which is provided with the MgF₂optical thin film as described above, is used for the optical system, itis possible to provide the optical system having the high opticalperformance in which the ghost and the flare are scarcely caused.

The extraordinary low refractive index layer is introduced into theconstruction of the multilayered antireflection film, and thearrangement of the extraordinary low refractive index layer and thearrangement of other layers are optimized. Accordingly, it is possibleto realize the multilayered antireflection film having the extremelyexcellent performance which cannot be realized by any conventionalmultilayered antireflection film. Therefore, the optical element and theoptical system of the present invention are extremely useful for avariety of ways of use including, for example, not only the opticalinstrument having the high resolution such as the camera, themicroscope, the binoculars, the exposure apparatus and the like but alsothe display such as the liquid crystal display device, the plasmadisplay and the like, the window glass,the show window, and the like.

1. A method for producing an MgF₂ optical thin film, comprising:preparing a sol solution, in which MgF₂ minute particles are dispersed,by mixing a magnesium compound and a fluorine compound in a solvent;placing the sol solution in a vessel which is tightly closable andperforming a pressurizing treatment and a heat treatment at the sametime; preparing a binder solution which contains a component capable offorming an amorphous silicon oxide-based binder by a reaction, theamorphous silicon oxide-based binder being composed of amorphous silica;preparing a coating liquid by mixing the sol solution for which thepressurizing treatment and the heat treatment have been performed andthe binder solution; forming a film by coating the coating liquid on abase material and by performing drying; and performing a heat treatmentafter forming the film, wherein the MgF₂ minute particles are connectedto one another by the amorphous silicon oxide-based binder.
 2. A methodfor producing an MgF₂ optical thin film, the method comprising:preparing a sol solution, in which MgF₂ minute particles are dispersed,by mixing a magnesium compound and a fluorine compound in a solvent;placing the sol solution in a vessel which is tightly closable andperforming a pressurizing treatment and a heat treatment at the sametime; preparing a binder solution which contains a component capable offorming an amorphous silicon oxide-based binder by a reaction, theamorphous silicon oxide-based binder being composed of amorphous silica;forming a porous film by coating the sol solution, for which thepressurizing treatment and the heat treatment have been performed, on abase material and by performing drying; coating the binder solution onthe porous film and impregnating the binder solution into the porousfilm; and performing a heat treatment after the impregnation, whereinthe MgF₂ minute particles are connected to one another by the amorphoussilicon oxide-based binder.
 3. The method for producing the MgF₂ opticalthin film according to claim 1 if wherein the magnesium compound ismagnesium acetate, the fluorine compound is hydrofluoric acid, and thesolvent is methanol.
 4. The method for producing the MgF₂ optical thinfilm according to claim 1, wherein a molar ratio of fluorine containedin the fluorine compound existing in the solvent to magnesium containedin the magnesium compound existing in the solvent is 1.9 to 2.0.
 5. Themethod for producing the MgF₂ optical thin film according to claim 1,wherein the component, which is capable of forming the amorphous siliconoxide-based binder, is an organic silicon compound.
 6. The method forproducing the MgF₂ optical thin film according to claim 5, wherein theorganic silicon compound is silicon alkoxide, a polymer thereof, orpolysilazane.
 7. The method for producing the MgF₂ optical thin filmaccording to claim 1, wherein an SiO₂-converted concentration of siliconin the coating liquid is not more than 5% by weight.
 8. The method forproducing the MgF₂ optical thin film according to claim 1, wherein thecoating liquid is coated on the base material by a spin coat method or adip coat method.
 9. The method for producing the MgF₂ optical thin filmaccording to claim 1, wherein the coating liquid is coated on the basematerial in an atmosphere of relative humidity of 5% to 40% by a spincoat method.
 10. The method for producing the MgF₂ optical thin filmaccording to claim 1, wherein the coating liquid is coated on the basematerial by a spin coat method by rotating the base material at amaximum number of revolutions of 500 rpm to 9,000 rpm within 3 secondsafter supplying the coating liquid to the base material.
 11. The methodfor producing the MgF₂ optical thin film according to claim 1, whereinthe MgF₂ optical thin film having a desired refractive index is producedby adjusting an SiO₂-converted concentration of silicon in the coatingliquid.
 12. The method for producing the MgF₂ optical thin filmaccording to claim 1, wherein a plurality of pieces of the MgF₂ opticalthin film having desired refractive index is produced by adjusting amolar ratio of fluorine contained in the fluorine compound to magnesiumcontained in the magnesium compound of the sol solution.
 13. The methodfor producing the MgF₂ optical thin film according to claim 1, whereinthe MgF₂ minute particles have an average particle diameter of 1 nm to100 nm.
 14. The method for producing the MgF₂ optical thin filmaccording to claim 2, wherein the magnesium compound is magnesiumacetate, the fluorine compound is hydrofluoric acid, and the solvent ismethanol.
 15. The method for producing the MgF₂ optical thin filmaccording to claim 2, wherein a molar ratio of fluorine contained in thefluorine compound existing in the solvent to magnesium contained in themagnesium compound existing in the solvent is 1.9 to 2.0.
 16. The methodfor producing the MgF₂ optical thin film according to claim 2, whereinthe component, which is capable of forming the amorphous siliconoxide-based binder, is an organic silicon compound.
 17. The method forproducing the MgF₂ optical thin film according to claim 6, wherein theorganic silicon compound is silicon alkoxide, a polymer thereof, orpolysilazane.
 18. The method for producing the MgF₂ optical thin filmaccording to claim 2, wherein an SiO₂-converted concentration of siliconin the binder solution is not more than 5% by weight.
 19. The method forproducing the MgF₂ optical thin film according to claim 2, wherein thesol solution is coated on the base material by a spin coat method or adip coat method.
 20. The method for producing the MgF₂ optical thin filmaccording to claim 2, wherein the sol solution is coated on the basematerial in an atmosphere of relative humidity of 5% to 40% by a spincoat method.
 21. The method for producing the MgF₂ optical thin filmaccording to claim 2, wherein the sol solution is coated on the basematerial by a spin coat method by rotating the base material at amaximum number of revolutions of 500 rpm to 9,000 rpm within 3 secondsafter supplying the sol solution to the base material.
 22. The methodfor producing the MgF₂ optical thin film according to claim 2, whereinthe MgF₂ optical thin film having a desired refractive index is producedby adjusting an SiO₂-converted concentration of silicon in the bindersolution.
 23. The method for producing the MgF₂ optical thin filmaccording to claim 2, wherein a plurality of pieces of the MgF₂ opticalthin film having desired refractive index is produced by adjusting amolar ratio of fluorine contained in the fluorine compound to magnesiumcontained in the magnesium compound of the sol solution.
 24. The methodfor producing the MgF₂ optical thin film according to claim 2, whereinthe MgF₂ minute particles have an average particle diameter of 1 nm to100 nm.